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Cellulosics dyeing by john shore

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  • 1. iii Cellulosics Dyeing 1995 Society of Dyers and Colourists Edited by John Shore Formerly of BTTG-Shirley and ICI (now Zeneca), Manchester, UK DANISH RAJA
  • 2. iv Copyright © 1995 Society of Dyers and Colourists. All rights reserved. No part of this publication may be reproduced, stored in a retrieval system or transmitted in any form or by any means without the prior permission of the copyright owners. Published by the Society of Dyers and Colourists, PO Box 244, Perkin House, 82 Grattan Road, Bradford, West Yorkshire BD1 2JB, England, on behalf of the Dyers’ Company Publications Trust. This book was produced under the auspices of the Dyers’ Company Publications Trust. The Trust was instituted by the Worshipful Company of Dyers of the City of London in 1971 to encourage the publication of textbooks and other aids to learning in the science and technology of colour and coloration and related fields. The Society of Dyers and Colourists acts as trustee to the fund, its Textbooks Committee being the Trust’s technical subcommittee. Typeset by the Society of Dyers and Colourists and printed by The Alden Press, Oxford. ISBN 0 901956 68 6
  • 3. v Contents Contributors viii Preface ix CHAPTER 1 Cellulose: structure, properties and behaviour in the dyeing process by Thomas P Nevell 1 1.1 Introduction 1 1.2 Cellulosic textile fibres 1 1.3 Molecular structure 14 1.4 Supramolecular structure 16 1.5 Degradation of cellulose 26 1.6 Derivatives 41 1.7 Affinity for water and organic liquids 48 1.8 Solvents for cellulose 52 1.9 Swelling 57 1.10 Structure of cotton in relation to dyeing 65 1.11 Structure of regenerated cellulosic fibres in relation to dyeing 70 1.12 References 73 CHAPTER 2 Preparation by William S Hickman 81 2.1 Introduction 81 2.2 Impurities present in cellulosic fibres 82 2.3 Chemicals for preparation 83 2.4 Water 90 2.5 Preparation machinery 91 2.6 Cotton preparation 106 2.7 Bleaching 117 2.8 Preparation of regenerated cellulosic fibres 136 2.9 Preparation of bast fibres 138 2.10 Monitoring the process liquors 140 2.11 Monitoring the results 142 2.12 Final comments 148 2.13 References 149
  • 4. vi CHAPTER 3 Dyeing with direct dyes by John Shore 152 3.1 Introduction 152 3.2 Structure and properties of direct dyes for cellulose 153 3.3 Dye structure and dyeability of cellulosic fibres 162 3.4 Application properties of direct dyes 171 3.5 Batchwise application of direct dyes to cellulosic textiles 175 3.6 Semi- and fully continuous dyeing processes for direct dyes 177 3.7 Aftertreatment processes for direct dyeings 177 3.8 Significance of finishing for direct dyes on cellulosic fabrics 182 3.9 References 186 CHAPTER 4 Dyeing with reactive dyes by John Shore 189 4.1 Introduction 189 4.2 Historical background 190 4.3 Reactive systems 192 4.4 Structure and properties 209 4.5 Batchwise application 214 4.6 Semi- and fully continuous application 225 4.7 Washing-off and aftertreatment 232 4.8 Stability of dye–fibre bonds 237 4.9 Reactive dyeing residues in waste liquors 241 4.10 References 242 CHAPTER 5 Dyeing with vat dyes by Francis R Latham 246 5.1 Introduction 246 5.2 Fundamental principles 247 5.3 Fundamental processes of vat dyeing 263 5.4 Dyeing methods 267 5.5 References 278 CHAPTER 6 Dyeing with sulphur dyes by Colin Senior 280 6.1 Commercial position 280 6.2 Constitution of sulphur dyes 281 6.3 Classification and commercial forms 282
  • 5. vii 6.4 Auxiliaries 285 6.5 Application methods 294 6.6 Effluent treatment 311 6.7 Fastness performance 313 6.8 Common faults 316 6.9 References 319 CHAPTER 7 Dyeing with azoic components by John Shore 321 7.1 Introduction 321 7.2 Chemical constitution and hues obtainable 322 7.3 Treatment with naphthols 326 7.4 Intermediate treatments 331 7.5 Development 333 7.6 Aftertreatment 340 7.7 Dyeing of mercerised cotton and other cellulosic fibres 342 7.8 Equipment for azoic dyeing 344 7.9 Fastness of azoic dyeings 349 7.10 Stripping of azoic dyeings 350 7.11 References 350 CHAPTER 8 Selection of dyes for dyeing cellulosic fibres by John Shore 352 8.1 Introduction 352 8.2 Principles of evaluation and testing of dyes 356 8.3 Influence of colour and dyeing properties on selection between dye classes 360 8.4 Selection between dye classes according to substrate and fastness requirements 376 8.5 Factors governing choice between sub-classes within the major dye classes 388 8.6 References 395
  • 6. viii Contributors Thomas P Nevell Formerly senior lecturer in polymer and fibre science, UMIST, Manchester, UK William S Hickman Application development manager, Solvay Interox R&D, Widnes, UK John Shore Formerly of BTTG-Shirley and ICI (now Zeneca), Manchester, UK Francis R Latham Business development, BASF plc, Colours and Specialities Division, Cheadle, UK Colin Senior Chief colourist, James Robinson Ltd, Huddersfield, UK
  • 7. ix Preface This book is another in the series on colour and coloration technology initiated by the Textbooks Committee of the Society of Dyers and Colourists under the aegis of the Dyers’ Company Publications Trust Management Committee, which administers the trust fund generously provided by the Worshipful Company of Dyers. It has been written to replace the previous work in this series entitled The dyeing of cellulosic fibres, which is now out of print. Assembled mainly about fifteen years ago under the editorship of Cliff Preston and eventually published in 1986, that multi-author book had become somewhat outdated. Rather than prepare a formal second edition, Textbooks Committee decided to go for a modified approach. A smaller team of authors was appointed with a view to completing the update more quickly and achieving a more coherent presentation of the contents. To take due account of technological changes in the dyeing of cellulosic materials over recent years, at least half of the book has been completely re- written and the remaining less active topics have been updated and rearranged considerably. Much new illustrative material has been added and the presentation of those illustrations retained from the 1986 book has been much improved. Discussion of the preparation and dyeing of cellulosic blend materials has been deliberately omitted, in view of the decision by Textbooks Committee to produce a separate publication on the dyeing of fibre blends. This present work has been completely re-indexed and the lists of references at the end of each chapter have been revised, extended and updated throughout. In many of the areas covered, of course, we have retained material or ideas originated by our fellow authors of the chapters in the 1986 book and we should like to record our sincere thanks to them for helping us in this way to build on their contributions. These individuals are: Ulrich Baumgarte, David Clarke, Ken Dickinson, Maurice Fox, Hans Herzog, Ian Holme, Bernard Kramrisch, Bill Marshall and Harry Sumner. The authors and editor of this book are indebted to our referees for valuable comments and suggestions for improvement. Our grateful thanks are due to Paul Dinsdale (the editor of the Society), Ros Amery (copy editing and proof reading), Carol Davies (artwork and layout) and Sue Bailey (typing). We are particularly impressed by the quality of the illustrations, which are a notable improvement on those in the 1986 book on this subject. JOHN SHORE
  • 8. 1 HomeContents 1 CHAPTER 1 Cellulose: structure, properties and behaviour in the dyeing process Thomas P Nevell 1.1 INTRODUCTION Cellulose is the most abundant of all naturally occurring organic polymers, thousands of millions of tonnes being produced by photosynthesis annually throughout the world [1]. Although exploited for several millennia in the forms of cotton, flax and other textile fibres, and in the form of wood for papermaking and constructional purposes, our knowledge of its chemistry is comparatively recent. It was first recognised in 1838 as the common structural material of many of the higher land plants by Payen, who invented the name cellulose [2]. However, it was not until the 1930s that its constitution as a linear high polymer of anhydroglucose units was unequivocally established [3,4]. This did not prevent much academic and industrial research from being done from the 1850s onwards. Notable technological advances were made long before they could be explained on a molecular basis, e.g. the development of viscose fibre manufacture at the turn of the century. The progress of research has not slackened – indeed it appears to be accelerating [5]. A vast and complicated literature has arisen which may well appear daunting. The purpose of this chapter is to describe the basic facts as they are understood at the present day. It owes much to several standard works on the subject [1,3,6–47]. 1.2 CELLULOSIC TEXTILE FIBRES 1.2.1 Cotton Cotton, the purest form of cellulose found in nature, is the seed hair of plants of the genus Gossypium [44,48]. Many species are grown commercially, but they may be conveniently divided into three types. Type 1 fibres have staple lengths
  • 9. 2 CELLULOSE: STRUCTURE, PROPERTIES AND BEHAVIOUR IN THE DYEING PROCESS HomeContents (i.e. average fibre lengths) varying from 25 to 60 mm, including high-quality fine cottons such as the Egyptian, Sudanese and Sea Island varieties. Type 2 are coarser species with shorter staple lengths (about 13–33 mm), such as American upland cottons, and type 3 species of still shorter staple lengths (about 9–25 mm), commonly produced in various Asian countries. Major cotton producers at the present time are the USA, China, India, Pakistan and the Central Asian republics; other countries producing small but not insignificant quantities include Mexico, Turkey, Brazil, Egypt and Sudan. The mature cotton fibre forms a flat ribbon varying in width between 12 and 20 µ m. It is highly convoluted, probably on account of the twisting that takes place when the tubular shape formed during growth collapses on drying. The number of convolutions varies between four and six per millimetre, reversing in direction every millimetre or so along the fibre. These characteristics make cotton easy to recognise under both optical and electron microscopes (Figures 1.1 and 1.2). Cotton fibres are also illustrated in cross-section in Figures 1.1 and 1.2. Their bean-shaped appearance is sometimes described as a bilateral structure, which indicates that the density of packing of cellulose chains is not uniform across the fibre. Hence the accessibility of the chain segments to various reagents varies across the fibre. Three main zones (A, B and C in Figure 1.3) have been identified by means of enzymic degradation [49]. The rate of degradation increases from A to B to C, which is the order of decreasing density of packing. Between zones A and C there appear to be limited areas (denoted by N) even more accessible then zone C. As well as ‘normal’ (i.e. mature) fibres, a typical batch of cotton may also Figure 1.1 Scanning electron micrographs of raw cotton fibres. (Source: BTTG-Shirley, Manchester.) 25 µm
  • 10. 3 HomeContents CELLULOSIC TEXTILE FIBRES Figure 1.2 Optical micrographs of raw cotton fibres × 184. (Source: Mr J T Jones, Dept of Textiles, UMIST, Manchester.) A A B C N N Figure 1.3 Bilateral structure of mature cotton (zones A, B, C and N differ in fibrillar packing density). (Source: [49].) contain some immature and dead fibres. A normal fibre may be defined as one that, after being swollen in aqueous sodium hydroxide, appears rod-like with no continuous lumen and no well-defined convolutions [50]. The lumen is what remains of the central canal from which the layers of cellulose were laid down in the secondary wall while the fibre was growing; it contains some residual protein. In dead fibres the wall thickness after swelling is one-fifth or less of the maximum ribbon width (usually found about midway between two
  • 11. 4 CELLULOSE: STRUCTURE, PROPERTIES AND BEHAVIOUR IN THE DYEING PROCESS HomeContents Figure 1.4 Idealised diagram of cotton morphology. (Source: J O Warwicker, in Liquid ammonia treatment of textiles, Shirley Institute, Manchester,1970.) , ,,,, Lumen Primary wall S3 S2 20–35o S1 20–30o Pectin Fats Waxes Reversal convolutions). Immature fibres are thin-walled because the secondary walls have not fully developed between their ceasing to grow in length and the bursting open of the boll. The boll is the pod remaining on the plant after the flower has fallen; it may contain 20 or more individual seeds. Cotton fibres have a fibrillar structure. Their morphology, illustrated schematically in Figure 1.4, exhibits three main features: primary wall, secondary wall and lumen. The primary wall consists of a network of cellulose fibrils covered with an outer layer, or cuticle, of pectin, protein, mineral matter and wax. The wax renders the fibre im- permeable to water and aqueous solutions unless a wetting agent is present. The secondary wall constitutes the bulk of a mature fibre and consists almost entirely of fibrils of cellulose arranged spirally around the fibre axis, the direction of the spiral reversing (i.e. changing between S and Z twists) many times along a single fibril. Reversal points frequently correspond with reversals of the exterior convolutions. The secondary wall consists of several layers and the spiral angle of the fibrils varies from one layer to the next, from about 20° to 35°. Although fibrils may be seen under a high-power micro- scope, there is considerable uncertainty about their dimensions. The resolution limit of the optical microscope is around 0.2 µ m, but the fibrils seem to be bundles of smaller microfibrils, which appear in the electron microscope to be 0.02–0.03 µ m thick and at least 10 µ m long. The microfibrils are themselves believed to consist of still smaller ‘elementary’ fibrils that are approximately 0.004 × 0.006 µ m in cross-section. Thus the cotton fibre consists of an assembly of fibrils in which the cellulose is
  • 12. 5 HomeContents ,,, , ,,,, ,,, B C C C A Figure 1.5 Idealised diagram of elementary fibrils in cotton. A, coalesced highly ordered surfaces; B, readily accessible disordered surfaces; C, readily accessible surfaces of tilt/twist regions. (Source: [53].) CELLULOSIC TEXTILE FIBRES accessible to most reagents only at fibrillar surfaces by way of a system of voids and channels [51,52]. The ‘crystalline-fibril’ concept of the fine structure of cotton has been further refined by Rowland and Roberts (Figure 1.5) [53]. Some surfaces (A) of the elementary fibrils are so close to one another that they are inaccessible to all chemical reagents unless swelling has been induced. Others (B) are relatively disordered and are more accessible; in addition there are readily accessible surfaces (C) in the tilt–twist regions that connect the more ordered regions. It is instructive to compare the composition of the primary wall with that of the cotton fibre as a whole, since it is the primary wall that becomes disintegrated when preparing cotton for dyeing and printing. Although the primary wall accounts for only 5% by weight of the fibre, it contains most of the non- cellulosic constituents. Table 1.1 gives comparative data for a typical cotton [54]. The ‘other substances’ are mostly water-soluble organic acids and sugars. The table illustrates the very high cellulose content of cotton, which surpasses all other natural sources in this respect.
  • 13. 6 CELLULOSE: STRUCTURE, PROPERTIES AND BEHAVIOUR IN THE DYEING PROCESS HomeContents Table 1.1 Composition of a typical cotton _________________________________________________ Proportion of dry weight/% ___________________________ Constituent Whole fibre Primary wall _________________________________________________ Cellulose 94.0 54.0 Protein (%N × 6.25) 1.3 14.0 Pectin 1.2 9.0 Wax 0.6 8.0 Ash 1.2 3.0 Other substances 1.7 12.0 _________________________________________________ 1.2.2 Bast fibres The bast fibres [45,46,55] are obtained from the stems of certain dicotyledonous plants, the most important of which is Linum usitatissimum, generally known as flax. They form a strength-giving protective layer around the woody central portion of the stem, and are themselves protected by an outermost cuticle which contains waxes and other substances. The fibres consist of bundles of thick- walled cells held together by gummy substances. The ultimate fibres vary in dimensions from one species to another. For example, in flax the length varies from 25 to 30 mm and the diameter from 15 to 35 µm. On the other hand, the individual cells of ramie are unusually long (about 150 mm), whilst their width (30–50 µm) is of the same order as that of flax. In the bast bundles the ends of the individual fibres overlap, forming continuous filaments extending along the length of the stem. Most bast fibres are used as full-length bundles, but flax is separated into its ultimate fibres for the production of fine linen yarns, and ramie is also similarly separated before spinning. The commercial use of ramie is limited, but the supramolecular structure of the fibre is of particular scientific interest. The microscopic features of flax, jute and ramie are illustrated in Figures 1.6–1.8. Separation of the fibre bundles from the harvested stems is complicated and time-consuming. Usually, the first stage is ‘retting’, i.e. soaking the stems for several weeks in water, traditionally in slow-moving rivers or bogs, until the effect of bacterial action on the intercellular material loosens the fibres sufficiently for mechanical separation. The treatment time can be reduced to a few days by retting in tanks at temperatures between 27 and 32°C. This applies particularly to the production of linen from flax. Subsequent mechanical
  • 14. 7 HomeContents CELLULOSIC TEXTILE FIBRES Figure 1.6 Scanning electron micrographs of flax. (Source: BTTG-Shirley, Manchester.) Figure 1.7 Scanning electron micrographs of jute. (Source: BTTG-Shirley, Manchester.) Figure 1.8 Scanning electron micrographs of ramie. (Source: BTTG-Shirley, Manchester.) 25 µm 25 µm 25 µm
  • 15. 8 CELLULOSE: STRUCTURE, PROPERTIES AND BEHAVIOUR IN THE DYEING PROCESS HomeContents separation is described as stripping (jute), or scutching and hackling (flax). The decortication of ramie is carried out without preliminary retting. The crude fibres contain far less cellulose than cotton, as may be seen from Table 1.2. The intercellular material consists of pectins, hemicelluloses (both complex polysaccharides) and lignin (a highly complex crosslinked polymer based on substituted phenylpropane units, found in large quantities in wood). Jute is frequently used without further purification, but flax and ramie are usually scoured and sometimes bleached. Much of the non-cellulosic material is removed, to an extent depending on the quality required in the product. Flax containing ‘sprit’ (remains of the woody core of the stem of the plant) may be difficult to dye level because the flax fibre and sprit differ in dyeability. The problem is similar to that of motes (remnants of seed coat) in raw cotton. Jute for dyeing is prescoured, but considerable quantities of lignin usually remain, leading to poor light fastness. Table 1.2 Composition of some typical bast fibres ___________________________________________________ Proportion of dry weight/% ______________________________ Raw Decorticated Constituent Jute flax ramie ___________________________________________________ Cellulose 71.3 80.1 83.3 Intercellular material 27.1 10.5 7.5 Wax 0.4 2.6 0.2 Ash 0.8 1.5 2.1 Other substances 0.4 5.3 6.9 ___________________________________________________ 1.2.3 Regenerated fibres The most abundant source of cellulose is wood, of which it constitutes about 40– 50%. Wood fibres are too short to spin into textile yarns but they are ideal for making paper, which is essentially a random mat of cellulosic fibres held together by hydrogen bonding. For wood fibres to be converted into textile yarns they must first be dissolved in a suitable solvent from which they can be regenerated as continuous filaments after the solution has been extruded through a fine orifice. The first such conversion was achieved by Chardonnet [47,56] around 1885. Cellulose was nitrated and dissolved in a mixture of diethyl ether and
  • 16. 9 HomeContents ethanol. The solution was extruded either into water (wet spinning) or into a hot-air chamber (dry spinning) to produce filaments of cellulose nitrate. Since these were highly inflammable they were denitrated with ammonium hydrosulphide. The final product was the first regenerated cellulose to enjoy any commercial success, but it is now obsolete. It has been superseded almost entirely by various types of viscose, although very small quantities of cupro (cuprammonium rayon) are still produced for special purposes. The main raw material from which regenerated fibres are manufactured is specially purified wood pulp. In the past, particularly for cupro, cotton linters have also been used. These are the very short cotton seed hairs left after those suitable for spinning directly into yarns have been separated. Although easier to purify, they were always more expensive than wood pulp. Cupro was first produced commercially in 1901, although it had been made experimentally some years earlier [31,56]. Linters or wood pulp are dissolved in cuprammonium hydroxide and the solution is extruded into water. The filaments are stretched and passed through dilute sulphuric acid to complete the regeneration of the cellulose. Viscose The viscose process [31,47,56] was discovered by Cross and Bevan in the last century, but it was not until 1910 that appreciable quantities of what is now called regular viscose were produced. Even then it was a comparative rarity. The viscose solution consists of sodium cellulose xanthate dissolved in aqueous sodium hydroxide. Cellulose filaments are obtained by extruding the solution through a spinneret into an aqueous bath containing sodium sulphate and sulphuric acid. The chemistry of the process will be described later (section 1.6.2). The filaments are stretched mechanically during regeneration and their properties are determined by the concentration of the viscose solution, the rate at which it is allowed to coagulate, the rate of stretching and the point at which stretching occurs during coagulation. The rate of coagulation is controlled by temperature and additives such as zinc sulphate or glucose and, for some types of fibre, small amounts of so-called ‘modifiers’, e.g. dimethylamine or cyclohexylamine [57,58]. Regular viscose differs from cotton in being non-fibrillar, having no lumen, and having a much lower degree of polymerisation (DP) (section 1.3). Although viscose filaments consist wholly of cellulose, their skin and core differ somewhat in supramolecular structure. The skin contains numerous small crystallites, whilst the core has fewer but larger crystallites [59] (section 1.4.3). The relative CELLULOSIC TEXTILE FIBRES
  • 17. 10 CELLULOSE: STRUCTURE, PROPERTIES AND BEHAVIOUR IN THE DYEING PROCESS HomeContents proportions of skin and core vary according to the conditions of coagulation. Under the microscope viscose fibres appear striated longitudinally and in cross- section their circumference is irregularly serrated due to the contraction of an originally cylindrical shape during the later stages of their formation (Figure 1.9). Modal fibres Compared with cotton, regular viscose suffers from the disadvantage of much lower breaking strength, particularly when wet. This is seldom a problem in apparel, but renders it unsuitable for more critical end uses. High-tenacity viscose fibres have been produced for at least half a century. They are highly oriented all- skin filaments with a near-circular cross-section (Figure 1.10). Originally developed for reinforcing cord in rubber tyres, they have been partly supplanted for this purpose by nylon and, more recently, polyester. The first high-tenacity fibres still had the disadvantage of a low wet modulus, and it was to overcome this that modified high wet modulus (so-called because of ‘modifiers’ in the spin bath) and polynosic fibres were introduced. The polynosic fibres were first developed in Japan [16,60]. The distinctive features in their method of production are: (1) a solution of viscosity higher than that used for regular viscose (achieved by using an aged pulp of higher DP rather than a more concentrated solution); (2) a spin bath of low salt concentration with no modifiers or other additives; and (3) a lower than usual spinning temperature. Figure 1.9 Optical micrographs of Evlan (a coarse crimped regular viscose fibre used for carpets) × 92. (Source: Mr J T Jones, Dept of Textiles, UMIST, Manchester.)
  • 18. 11 HomeContents CELLULOSIC TEXTILE FIBRES Under these conditions, high stretch (up to 300%) can be obtained. The product is a highly oriented fibre much more like cotton than other viscose fibres. Polynosic fibres have stress–strain curves similar to those of cotton and are largely unaffected by dilute sodium hydroxide solutions (up to 8%), which dissolve as much as 25% of regular viscose. They appear to be fibrillar in structure. More details of the preparation and properties of regenerated fibres are available in several reviews [16,31,47,61]. The term modal has been introduced to embrace all regenerated viscose fibres having tenacities in the conditioned state and wet moduli at 5% extension above certain defined values. Textile terms and definitions [62] defines a modal fibre as one satisfying conditions (1.1a) and (1.1b): Bc ≥ 1.3 T + 2T (1.1a) Bm ≥ 0.5 T (1.1b) where Bc is the breaking strength in the conditioned state (in cN), Bm is the force required for 5% extension in the wet state (in cN) and T is the linear density (in dtex). For a single filament of linear density 1.7 dtex these formulae give a minimum tenacity of 30 cN tex–1 and a minimum wet modulus at 5% extension of 76 cN tex–1. Figure 1.10 Optical micrographs of modal fibres × 184. (Source: Mr J T Jones, Dept of Textiles, UMIST, Manchester.)
  • 19. 12 CELLULOSE: STRUCTURE, PROPERTIES AND BEHAVIOUR IN THE DYEING PROCESS HomeContents Miscellaneous speciality viscose fibres Numerous variants of regular viscose have been developed for special purposes. A few of these will now be mentioned briefly. Chemically crimped viscose is produced by modifying the regeneration conditions so that the skin of the filament ruptures while in the spin bath [63]. The liquid viscose exposed is thus regenerated under different conditions from the skin, and a bicomponent structure results. A permanent crimp develops as a result of the differential shrinkage in subsequent washing and drying. In the wet processing of such fibres, excessive tension results in loss of crimp and should be avoided. Chemically crimped viscose is usually used in staple form and has an attractive handle. Hollow fibres have been produced in an attempt to simulate the lumen of cotton [64–70]. One method of doing this is to extrude viscose solution containing sodium carbonate into the acid spin bath. The carbon dioxide produced inside the filament creates a continuous channel, similar to the lumen in cotton but usually larger. Hollow viscose fibres have a lower density (1.15 g cm–3) than regular viscose (1.52 g cm–3) and a higher water imbibition value (130% compared with 90%). They provide greater bulk and fabric cover than regular viscose at the same fabric density and have a soft handle like that of combed cotton. By a careful choice of spinning conditions it is possible to produce hollow fibres with water imbibition values as high as 200% – the so-called super- absorbent fibres designed for surgical and sanitary purposes [71,72]. An alternative approach [73,74] is to add water-retentive polymers such as sodium polyacrylate or sodium carboxymethylcellulose to the viscose solution before spinning. Flame-retardant fibres have been produced by dispersing a suitable agent in the viscose solution before spinning [75,76]. One of the most successful agents was tris(2,3-dibromopropyl) phosphate (used in such fibres as Darelle), but in the late 1970s it was suspected of being carcinogenic and had to be abandoned. Little flame-retardant viscose is now manufactured for apparel, although some intended for other uses, containing flame retardants that are less hazardous, is still produced. Viscose fibres of modified dyeability have been produced for use in the dyeing of wool/viscose blends [77,78]. Additives with free amino groups incorporated into the viscose solution resulted in fibres that could be dyed with acid dyes. Although effective they were too expensive for commercial exploitation. Producer-coloured viscose is known as ‘dope-dyed’ yarn because insoluble pigments are dispersed in the viscose ‘dope’ from which it is spun. It exhibits
  • 20. 13 HomeContents excellent fastness to both light and laundering, but represents only a tiny fraction of fibre production. Regeneration from amine oxide solution: Tencel (Courtaulds) One of the most exciting recent developments in the production of regenerated cellulosic fibres has been the introduction of an alternative to the viscose process. The objectionable effects of viscose manufacturing plants on the environment have been universally recognised for a long time, but it was not until the 1970s that the possibility of using some other solvent for wood pulp began to emerge. The ‘new’ non-aqueous solvents for cellulose will be described later (section 1.8.3). The only one that so far has proved suitable for development is N-methyl- morpholine-N-oxide (NMMO). Satisfactory fibres have been regenerated from it on a small scale from time to time [79,80], but now Courtaulds PLC has established full-scale manufacture [81]. Wood pulp is dissolved in NMMO monohydrate and the solution extruded into dilute aqueous NMMO, with appropriate stretching and drying of the regenerated fibre. The solvent is totally recoverable for reuse so that the process is environmentally innocuous. The fibre is called Tencel (Courtaulds) and is illustrated in Figure 1.11. It is fibrillar in structure and approximates to cotton in its behaviour under stress and capacity for absorbing liquid water even more closely than modal fibres. Because of the close similarity of its stress–strain curve to that of cotton it can contribute to the CELLULOSIC TEXTILE FIBRES Figure 1.11 Optical micrographs of Tencel fibres × 184. (Source: Mr J T Jones, Dept of Textiles, UMIST, Manchester.)
  • 21. 14 CELLULOSE: STRUCTURE, PROPERTIES AND BEHAVIOUR IN THE DYEING PROCESS HomeContents strength of yarns blended from the two fibres, even at low blend levels. The chief mechanical properties and water imbibition of cotton, regular viscose, modal and Tencel fibres are compared in Table 1.3. Table 1.3 Comparison of typical fibre propertiesa ______________________________________________________________ Fibre US Middling Regular (titre 1.7 dtex) cotton viscose Modal Tencel ______________________________________________________________ Tenacity/cN tex–1 22 24 35 40 Elongation/% 8 22 14 15 Wet tenacity/cN tex–1 28 13 20 36 Wet elongation/% 13 27 14 17 Wet modulus at 5% extension/cN tex–1 100 50 110 270 Water imbibition/% 50 90 75 65 ______________________________________________________________ a From Courtaulds Tencel Technical Information pamphlet. 1.3 MOLECULAR STRUCTURE The main features of the chemical structure of cellulose are well known [30,82]. It may be most conveniently described as a 1,4-β-D-glucan, i.e. a condensation polymer of β-D-glucopyranose with 1,4-glycosidic bonds. The pyranose rings are in the 4C1 conformation (Figure 1.12(a)). For the sake of clarity and in accordance with convention the ring carbon atoms and the hydrogen atoms attached to them are omitted. Some authors also omit the C–H bonds. Hydroxy groups are always, and methylol groups are sometimes, printed in full. Although the Haworth projections formula (Fig 1.12(b)) is easier to write quickly it obscures some important stereochemical aspects of the glucopyranose ring [83]. Thus all the substituents, including the glycosidic bonds, are in the mean plane of the ring (equatorial) and not perpendicular to it (axial) as might be supposed from the Haworth formula. Only the C–H bonds are axial. The essential features of the polymer chain are the main sequence of intermediate units (I), the non-reducing end group (II), the reducing end group (III) and glycosidic linkages. The intermediate units possess one primary and two secondary alcohol groups each. The non-reducing end group possesses one extra secondary alcohol group at C4, and the reducing end group (so-called because it reduces Fehling’s solution and ammoniacal silver nitrate) is a cyclic hemiacetal. It
  • 22. 15 HomeContents MOLECULAR STRUCTURE Figure 1.12 Cellulose: (a) fully extended conformational formula; (b) the Haworth projection formula. n = degree of polymerisation (DP). HO O HO HO OH HO HO O O OH O O HO HO OH O HO OH OH CH2OH O OH OH CH2OH O O OH OH OH CH2OH O n–2 (a) n–2 II I III (b) OH CHOH O OH OH CH2OH O CHO OH OH OH CH2OH O Scheme 1.1 exhibits the characteristics of both an alcohol and an aldehyde under appropriate conditions (Scheme 1.1). The degree of polymerisation (DP) of cellulose varies with its source and is usually expressed as an average, since a wide distribution is found in most samples. In native cellulose it may be as high as 14 000 [32,84], but purification involving treatment with alkali usually reduces this to about 1000–2000. Furthermore, the hemiacetal end groups are converted to acid groups (section 1.5.3); thus purified native cellulose is normally devoid of reducing power. The DP of cellulose regenerated by the older methods is about 250–300, but that of modal fibres is higher (about 500–700). All fibres regenerated from viscose solution contain a very small number of aldehyde, ketone and carboxylic acid groups introduced during manufacture. Thus chemically, regenerated cellulose differs from native cellulose only slightly. Differences in properties arise mainly from differences in supramolecular structure.
  • 23. 16 CELLULOSE: STRUCTURE, PROPERTIES AND BEHAVIOUR IN THE DYEING PROCESS HomeContents 1.4 SUPRAMOLECULAR STRUCTURE 1.4.1 Introduction Cellulose is a highly crystalline material, but it does not form discrete crystals like those of glucose from which it is derived. As already indicated, native cellulose fibres consist of crystalline fibrils varying in complexity and length. This fibrillar structure is interspersed with material in which the chain molecules are less well ordered than in a crystal. The same chain molecule may participate in crystalline and less crystalline material along different portions of its length. Native cellulose contains very little genuine non-crystalline material. What used to be regarded as amorphous is generally believed to be the accessible surfaces of some of its crystalline fibrils (section 1.4.3). Regular viscose and cupro fibres do seem to contain some genuinely amorphous material, but modal fibres are essentially fibrillar in nature. The supramolecular structure of cellulose, i.e. the overall arrangement of chain molecules in a fibre, may be considered under two headings: crystalline structure and fine structure. 1.4.2 Crystalline structure [17–19,33,44] Five allomorphic forms of cellulose (often incorrectly referred to as polymorphic forms) have been identified [33], but only cellulose I and cellulose II are important in textile processing. Cellulose I is the form found in nature, and cellulose II (formerly, and misleadingly, called ‘hydrate cellulose’) is the thermodynamically stable form produced when cellulose is regenerated from solution or subjected in the solid state to the process of mercerisation (section 1.9.1). The main features of the crystal structures of the two forms were established in the 1930s by X-ray analysis [85,86]. They have been frequently refined during the intervening years, but some uncertainties in their precise details remain [33]. X-ray analysis alone proved insufficient to define the unit cells of cellulose I and II precisely, and more detailed information was obtained with the aid of electron diffraction and especially i.r. spectroscopy. The latter was used by Liang and Marchessault [87] to study the hydrogen-bonding system in cellulose I and this enabled them to conclude that the so-called ‘bent-chain’ model originally proposed by Hermans [88] was nearer the truth than the Meyer and Misch model. In the latter, the glycosidic bonds are in a plane perpendicular to the mean planes of the adjacent rings. In Hermans’ model, however, they are inclined at a small angle alternately positive and negative to these planes. The chain molecules are thus fully extended to the form of flat ribbons having the minimum possible
  • 24. 17 HomeContents SUPRAMOLECULAR STRUCTURE thickness in the direction perpendicular to the mean planes of the rings. It should be possible to visualise this structure from Figure 1.12(a), but it is much easier to do so with the aid of skeletal atomic models. The flat ribbons just described pack together into a monoclinic unit cell of dimensions approximately a = 0.82, b = 0.79, c = 1.034 nm (fibre axis) and γ = 97° in the case of cellulose I. A cross-section of the cell in the ab plane, looking along the fibre axis, is shown in Figure 1.13(a). Originally Meyer and Misch believed that the centre and corner chains were antiparallel to one another, but the latest evidence [33,89] strongly favours the parallel arrangement with the centre chains staggered by approximately 0.25c (0.26 nm) with respect to the corner chains, as in Figure 1.14(a). Adjacent chains are held together in a system of hydrogen-bonded sheets in the ac planes as shown in Figure 1.15, alternate sheets being staggered with respect to each other in accordance with the arrangement indicated in Figure 1.14(a). Thus there are two interpenetrating sets of sheets, one comprising the corner chains and the other the centre chains. There is no hydrogen bonding between corner and centre chains. Furthermore, the surfaces of the sheets are hydrophobic, which means that they are held together by van der Waals forces. Three features of the structure of cellulose I should be emphasised. Firstly, the chains are fully extended. Thus the repeat distance in the chain direction is equal to the length of two anhydroglucose units, i.e. one anhydrocellobiose unit. The degree of polymerisation is the average number of anhydroglucose units per chain molecule. Secondly, the unit cell parameters are stated according to the modern crystallographic convention for fibrous polymers [90,p.713]. Unfortun- Figure 1.13 Cross-section of unit cell in the ab plane (looking along the fibre axis): (a) cellulose I; (b) cellulose II). (Source: [89].)
  • 25. 18 CELLULOSE: STRUCTURE, PROPERTIES AND BEHAVIOUR IN THE DYEING PROCESS HomeContents Figure 1.14 Unit cell in the ac plane: (a) parallel-chain model of cellulose I; (b) antiparallel- chain model of cellulose II. (Source: [89].) a c a c (a) (b) Figure 1.15 Hydrogen bonding in cellulose I. (Source: [89].) a c
  • 26. 19 HomeContents SUPRAMOLECULAR STRUCTURE ately in all the older papers, and even in some (e.g. [44]) of the current literature of cellulose structure, the fibre axis has been identified with the b direction; with this usage the parameters of the unit cell would be a = 0.82, b = 1.034 (fibre axis), c = 0.79 nm and β (180° – γ ) = 83°. Care must be taken to avoid confusion between the two conventions and it is much to be hoped that the older will soon die out. Lastly, the unit cell dimensions are quoted with a lower degree of precision than X-ray analysis is capable of achieving. This is because they are averages of values that may differ by as much as 1% between samples of cellulose from different sources. The unit cell of cellulose II is also monoclinic, its average dimensions being a = 0.81, b = 0.91, c = 1.034 nm and γ = 117°. A cross-section in the ab plane is shown in Figure 1.13(b). The chain arrangement differs from that in cellulose I in two respects. Firstly, the planes of the glucose rings are inclined at a small angle to the ac plane. Secondly, the centre chains are oriented antiparallel to the corner chains. They are still staggered with respect to the corner chains in the c direction in the same way as in cellulose I, as Figure 1.14(b) shows. However, the hydrogen-bonding system is rather more complicated than in cellulose I. Not only are there hydrogen bonds in the ac planes between parallel corner chains and between parallel centre chains, but there are also hydrogen bonds between antiparallel corner chains and centre chains. Detailed illustrations of the system have been published [89], along with X-ray evidence for the antiparallel orientation of alternate chains. Such an arrangement would be expected in regenerated cellulose since there is no reason to suppose that the molecules would be other than randomly oriented in solution. It is more difficult to reconcile with the fact that cellulose I is converted into cellulose II by mercerisation (a process that does not involve dissolution of the material) but a plausible mechanism for the process has been proposed [33,91]. Chain folding in cellulose has been postulated [17,18], but the existence of a parallel-chain structure effectively disposes of the suggestion for cellulose I, since it is incompatible with any reasonable picture of the synthesis of cellulose in nature. It would also be difficult to reconcile with the low extensibility of native cellulosic fibres. The antiparallel arrangement in cellulose II is compatible with chain folding but it seems unlikely that the two forms would differ, especially since some regenerated cellulosic fibres have properties similar to those of cotton. Rejection of the folded-chain hypothesis for cellulose in fibrous form does not mean that chain-folded structures may not be formed under special conditions. For example, single crystals of cellulose triacetate containing folded chains can be obtained from solution in a mixture of nitromethane and butan-1-ol, and it is possible that some of the folds survive
  • 27. 20 CELLULOSE: STRUCTURE, PROPERTIES AND BEHAVIOUR IN THE DYEING PROCESS HomeContents saponification [92]. Similarly, under suitable conditions, cellulose of moderate DP can be crystallised from solution in aqueous sodium hydroxide in folded- chain form [93]. Cellulose III is not itself important in textile applications, but is mentioned here because it sometimes results from treating cellulose with liquid ammonia or aliphatic amines (section 1.9.3). It has a monoclinic unit cell with the following dimensions: a = 0.77, b = 1.00, c = 1.03 nm and γ = 122° [19]. 1.4.3 Fine structure A knowledge of the fine structure of cellulose is essential for a full understanding of dyeing processes since it is important to know precisely where a dye becomes attached to the fibre. It is generally accepted [77,94] that reagents cannot penetrate the crystalline regions unless they can simultaneously disrupt them. The nature of the less ordered, and therefore more accessible, regions of cellulosic fibres will now be discussed. The subject is still somewhat speculative. As long ago as 1967 Statton [95] pointed out that theories and models of fine structure are no more than theories and models, and broadly speaking the same is true today. Nevertheless, these concepts are helpful in considering the dyeing of fibres. Howsmon and Sisson [7] had earlier visualised a range of degrees of order in the packing of chain molecules in a fibre, as illustrated in Figure 1.16. In this picture there is a continuous transition from perfectly crystalline to completely amorphous material, but it should not be assumed that such a transition is characteristic of all fibres. It is merely a hypothetical representation of what might exist. In the case of cellulosic fibres the idea of fringed micelles, in which crystalline micelles are embedded in an amorphous matrix with individual chain molecules extending through several crystalline and amorphous regions, as in Figure 1.17(a), held sway for a long time. This subsequently gave way to the concept of fringed fibrils, in which the fringe molecules that constitute the non-crystalline Perfect crystal Paracrystalline Amorphous with correlation Wholly amorphous Figure 1.16 Diagram of packing of chain molecules (amorphous to crystalline). (Source: [7].)
  • 28. 21 HomeContents SUPRAMOLECULAR STRUCTURE Figure 1.17 Diagram of fine structure in cellulose: (a) fringed micelles; (b) fringed fibrils. (Source: [44].) (a) (b) regions emerge from various points along crystalline fibrils, as in Figure 1.17(b), rather than from the ends of brick-like micelles. These and other theories of fibre structure have been discussed by Hearle [96]. The fringed fibril concept is still applicable to many regenerated cellulosic fibres but the crystalline fibril concept (section 1.2.1) is usually reserved for cotton and other native fibres. Ideally, the structure of a textile fibre should be capable of description in terms of the following five factors [75,97]: degree of order; degree of localisation of order and disorder; the length:width ratio of localised units; the size of localised units; and the degree of orientation of localised units. In practice these parameters are seldom capable of precise determination; measurements of accessibility (by chemical methods) and degree of crystallinity, orientation and crystallite size (by physical methods) are the most commonly encountered. Physical methods of determining crystallinity X-ray diffraction, i.r. spectroscopy and density measurements are the most important physical techniques for determining degree of crystallinity [98]. The X-ray method was pioneered by P H Hermans [99,100] and by Kast and Flaschner [101]. In essence it consists of separating the diffraction pattern of a fibre into its two components: that deriving from the crystalline and that from
  • 29. 22 CELLULOSE: STRUCTURE, PROPERTIES AND BEHAVIOUR IN THE DYEING PROCESS HomeContents the non-crystalline regions of the cellulose. The original i.r. method of Mann and Marrinan [102] aimed at determining the proportion of hydroxy groups that did not exchange with deuterium oxide vapour during exposure for one hour, those that did being considered to be in the amorphous regions. This was done by comparing the spectra of the original and deuterated samples. A procedure not involving deuterium exchange has also been developed [20,103]. The density method [104] is based on the observation that the density of cellulose ‘crystallites’ (obtained by the vigorous acid hydrolysis of cotton) is 1.545 whereas that of amorphous cellulose (obtained by ball milling) is 1.505. The crystallinity is expressed as the ratio of the difference between the specific volumes of amorphous cellulose and the sample, and the difference between the specific volumes of amorphous and crystalline cellulose. Detailed descriptions of these physical techniques are outside the scope of this chapter. Chemical methods of determining accessibility Chemical methods of studying the fine structure of cellulose have been reviewed by Rowland and Bertoniere [34]. The measurement of accessibility has been popular with cellulose chemists for many years because it is simpler and less expensive than the physical methods for crystallinity. Ideally, A = 100 – C where A is the percentage accessibility and C the percentage crystallinity. However, this simple relation is at best an approximation. Essentially, the measurement of accessibility involves either measuring the equilibrium absorption of some species such as water or iodine, or measuring the rate at which cellulose is attacked by a chosen reagent. Equilibrium uptake is usually readily obtainable under convenient conditions. The rate method almost always entails distinguishing the rapid initial rate at which the accessible regions react from the slower rate characteristic of the crystalline regions. These reactions are frequently of the first order, so a semi-logarithmic time plot yields a straight line that can be extrapolated to zero time to give the ‘crystallinity’. The trouble with chemical methods is that, although different reagents usually rank substrates in the same order, they give widely differing results for a given substrate. The reasons for this are fairly obvious. Firstly, if the fine structure of a fibre is relatively unaffected by the chosen reagent, the number of reaction sites accessible to the reagent must inevitably depend upon the size of the molecules. Secondly, if the fine structure can be altered by swelling or otherwise during the reaction the effects of different reagents will not be the same. One of the most frequently used reactions for assessing accessibility has been acid hydrolysis. In one procedure, weighed samples of material are boiled with
  • 30. 23 HomeContents 4–6 M HCl for various times up to about 24 h [105]. Increasing quantities of material dissolve (section 1.5.2) and the proportions of undissolved residue are plotted logarithmically against time; Figure 1.18 illustrates how accessibility may be estimated. One drawback of this method is that some extra crystallisation seems to occur during the early stages of hydrolysis [106], so that the estimates of accessibility may be on the low side. It may be noted that in practice co + ao 100. If all that is required is to rank substrates in order of accessibility, by far the simplest method is to compare their moisture regain values (section 1.7.1). Valentine has shown that the sorption ratio (SR), defined as the ratio of the moisture regain of the sample to that of purified cotton under standard conditions of humidity and temperature, is directly proportional to the fraction of non-crystalline material (F or FAM ), as measured by the i.r. method of Mann and Marrinan: SR = 2.60FAM [98]. For many purposes this is adequate. Nevertheless, a more detailed picture of the fine structure of cellulose can be obtained by what has been termed chemical microstructural analysis (CMA) by workers at the Southern Regional Research Center of the US Department of Agriculture, who have used it extensively [34]. The method was pioneered at the Shirley Institute [51,107,108] and entailed the methylation of cotton by a succession of treatments with diazomethane in water-saturated ether. Total hydrolysis of the products yielded the seven possible mono-, di- and tri- methylglucoses and their proportions were measured by gas–liquid chromatography. Hence the relative accessibilities of the three hydroxy groups in SUPRAMOLECULAR STRUCTURE Figure 1.18 Hydrolysis of regular viscose in 4 M HCl at 100°C. Extrapolation of curves gives the amorphous (ao) and crystalline (co) fractions in the untreated fibre. (Source: [105].) 0 8 16 2 3 4 6 8 10 20 40 60 100 Time/h Residue/% Crystalline fraction (co = 62%) Amorphous fraction (derived values) (ao = 24%)
  • 31. 24 CELLULOSE: STRUCTURE, PROPERTIES AND BEHAVIOUR IN THE DYEING PROCESS HomeContents each anhydroglucose unit were determined and conclusions were drawn about the structure of the elementary fibrils. Rowland and coworkers [34,109] did similar experiments with N,N-diethylaziridium chloride (DAC), which introduces N,N- diethylaminoethyl groups, (C2H5)2N–CH2CH2–, into the cellulose, and have suggested that these methods may well provide an insight into the supramolecular structure of cellulose that is not forthcoming from physical measurements. Orientation In the context of fibre structure, the term ‘orientation’ means the average alignment of either chain molecules in non-crystalline materials or crystallites in partially crystalline materials with respect to the fibre axis. Methods of measurement will be discussed only briefly here; more detailed accounts are readily available [44,98,110,111]. The earliest, and perhaps simplest, method was to measure the optical birefringence, i.e. the difference between refractive indices for light polarised parallel and perpendicular to the fibre axis. Hermans [104] has called the ratio of this difference to the difference for a perfectly oriented specimen the optical ‘orientation factor’ f, and has shown that for specimens of equal density it is given by Eqn 1.2, where θ is the average angle at which molecular chains are inclined to the fibre axis: f = 1 − 3 2 sin 2 θ (1.2) Polarised i.r. spectroscopy [112] has also been used to measure the orientation factor. The method depends on the absorption of radiation by a particular chemical group in the chain molecule. Only groups that behave as oscillating dipoles absorb i.r. radiation and only the component of the oscillations parallel to the electric vector is effective. Thus, if polarised radiation is used, maximum absorption will occur when the dipole vibrates parallel to the electric vector (i.e. at right angles to the plane of polarisation) and absorption should be zero perpendicular to the electric vector. It must be remembered, of course, that the vibrations of a particular group in a fully extended chain molecule are unlikely to be parallel to the axis of the chain. In practice, the ratio of the intensities of a chosen polarised i.r. absorption band parallel and perpendicular to the fibre axis is taken as a measure of the average angle between chains and the axis. Although it is quite a powerful method, Krässig [112] prefers the X-ray method, particularly for regenerated fibres, because of its simplicity and reliability. The X-ray diffraction pattern of a perfect crystal consists of an array of sharp
  • 32. 25 HomeContents SUPRAMOLECULAR STRUCTURE Figure 1.19 Procedure for measuring 50% X-ray angle. (Source: [111].) Pole Pole Pole Intensity Equator Degrees of rotation Pole Equator θ Fibre pattern Photometer trace X-ray angle = q/2 θ θ spots, but with a fibre the reflections nearly always take the form of arcs. The lengths of the arcs are directly related to the average orientation of the crystallites with respect to the fibre axis [113]. Short arcs indicate a well-oriented material, long arcs a less well-oriented material, and complete circles (as in a powder diagram) random orientation. The commonest procedure is to select one of the equatorial arcs and to measure the angles at which the intensity is 50% of the maximum value. The arithmetic mean of these two angles is known as the ‘50% X-ray angle’ as illustrated in Figure 1.19. From its value for two different reflections it is possible to calculate the average angle θ between the chains and the fibre axis. The X-ray orientation factor may then be obtained from Hermans’ equation (Eqn 1.2). The orientation factor of ramie (the most highly oriented natural cellulosic fibre) has been found to be 0.97, and that of regular viscose rayon 0.79, by this method [104]. Crystallite size [110,111] Whereas the length of arcs in an X-ray fibre diagram is determined mainly by the orientation of the ordered material, their width depends on the dimensions of the crystallites. The smaller the crystallite, the broader the X-ray reflection, in accordance with Scherrer’s equation [90,114] (Eqn 1.3): D = 0.9λ β cosθ (1.3)
  • 33. 26 CELLULOSE: STRUCTURE, PROPERTIES AND BEHAVIOUR IN THE DYEING PROCESS HomeContents where D is the dimension of the crystallite perpendicular to the diffracting plane, β the broadening of the arc in radians, λ the wavelength of the X-rays and θ the Bragg angle. The correlation is by no means perfect because other factors influence the arc broadening to some extent, but this method is probably the one most often used to estimate crystallite size. It depends on the same wide-angle scattering as is used for a determination of unit-cell dimensions and can be applied to both the length and the width of crystallites. Another technique, based on low-angle scattering, has been developed, but it is difficult experimentally and its theoretical basis is somewhat uncertain. A more direct method of estimating crystallite size is to break the material down into its crystallites and examine it in the electron microscope. This is more successful for native than regenerated cellulosic fibres. One difficulty is to be certain that the entities isolated by disintegration of the material are the same as the crystallites originally present. Chemical methods such as chemical microstructural analysis (p.23) have been used effectively, and the length of crystallites has sometimes been identified with the levelling-off DP when the material is subjected to acid hydrolysis (section 1.5.2). 1.5 DEGRADATION OF CELLULOSE 1.5.1 Introduction An industrially important feature of cellulosic fibres is chemical stability, enabling them to withstand degradation with its consequential loss of tensile strength under normal conditions of use. Even slight degradation during processing, however, may be accompanied by unacceptable loss of strength and other undesirable effects. The study of degradation, therefore, has important implications for satisfactory dyeing and finishing. Six different degradative agencies have been identified: acids, alkalis, oxidising agents, enzymes, heat and radiation [30]. Under certain circumstances cellulose can also be degraded mechanically, but this is never a problem in industrial processing. Ultimately the complete degradation of cellulose yields carbon dioxide and water. However, it is the early stages of partial degradation that are important in the textile field. Only slight changes in composition may affect the physical properties of cellulose profoundly, sometimes even reducing it to powder. The water-insoluble products of the action of acids and oxidising agents are still frequently referred to by the traditional trivial names of ‘hydrocellulose’ and ‘oxycellulose’ respectively [115–117]. The constitutions of partially degraded celluloses, though difficult to
  • 34. 27 HomeContents determine precisely and unequivocally, have been greatly illuminated by the methods perfected (though not originated) by Clibbens and his collaborators [118]. To study the effects of a particular degrading agent, a series of progressively modified materials is prepared and characterised by measuring properties related to DP, such as tensile strength and fluidity in cuprammonium hydroxide (section 1.8.2), and to content of carboxy and carbonyl groups. Methods selected from the enormous variety available have been described in detail [119]. Except for the determination of uronic acid groups by decarboxylation with boiling 12% hydrochloric acid [120], all methods of determining carboxy groups depend on exchange between cations in solution and the solid carboxy- containing material in its free-acid form. The essential features of each method are the means adopted to ensure that the equilibrium in Scheme 1.2 is as far to the right as possible, and the analytical procedure for measuring the quantity of cations exchanged. DEGRADATION OF CELLULOSE Several types of carbonyl-containing group may be found in chemically degraded cellulose, notably aldehyde, ketone and hemiacetal groups. For many years the only method of assessing them was the empirical measure known as the copper number. This was first introduced by Schwalbe [121] and is defined as the weight in grams of Cu(II) in alkaline solution reduced to Cu(I) at the boiling point by 100 g of dry material. The amount of Cu(II) reduced per carbonyl group varies greatly with the precise nature of the group and its position in the chain molecule. For example, the end group in a hydrocellulose reduces 22 atoms of copper [122]. Thus the copper number is a highly sensitive measure of reducing groups, even though it can seldom be related stoichiometrically to the content of such groups. It can provide valuable confirmation that a substrate is devoid of reducing groups; thus the copper number of scoured cotton is normally close to zero. Methods involving condensation reactions with hydroxylamine or sodium cyanide, or reduction to alcohol groups with sodium borohydride, have been fully described [119] and are claimed to give stoichiometric results. 1.5.2 Degradation by acids The degradation of cellulose by aqueous acids entails acid-catalysed hydrolysis of the glycosidic linkages according to Scheme 1.3 [123,124]. Rcell COOH + M+ Rcell COOM + H+ Scheme 1.2
  • 35. 28 CELLULOSE: STRUCTURE, PROPERTIES AND BEHAVIOUR IN THE DYEING PROCESS HomeContents O OH OH CH2OH O O OH OH CH2OH O OH OH CH2OH O O OH OH CH2OH O OH OH CH2OH O HO O OH OH CH2OH O OH OH CH2OH O HO O OH OH CH2OH OH2 (4) H,OH O OH OH CH2OH + H3O+ + H2O O HO O OH OH CH2OH (3) (1) + + 2H2O (2) + H3O + 2H2O + H2O O O H + + H Scheme 1.3 The properties of the hydrocellulose so formed depend solely on the number and distribution of glycosidic linkages broken, which vary with acid concentration, temperature and time of treatment. The identity of the acid is irrelevant and thus the tensile strength, copper number and fluidity of a hydrocellulose are uniquely related to one another, irrespective of how the hydrocellulose has been made [125]. The products of the early stages of
  • 36. 29 HomeContents DEGRADATION OF CELLULOSE hydrolysis are fibrous, but those with fluidities above about 44 rhes are usually powders. The rate of increase of fluidity depends on the supramolecular structure of the cellulose (Figure 1.20). Eventually the fluidity reaches a plateau, at a level depending on the nature of the starting material [126]. The degree of polymerisation calculated from one of these plateaux is often called the levelling- off degree of polymerisation (LODP). Even when the DP has become constant, the material continues to lose weight if the acid treatment is continued. Once the readily accessible bonds have been randomly hydrolysed, further reaction occurs at chain ends in the previously inaccessible regions of the fibres (often regarded as the ‘crystallites’). This slow terminal hydrolysis removes small stable fragments from chain ends and produces no noticeable effect on the average DP. It has been shown that, if the accessible regions involved in the first stage of the reaction are randomly distributed along the microfibrils of the original cellulose, an exponential distribution of crystallite lengths will result from hydrolysis and a constant particle-size distribution will be maintained as terminal degradation continues [21,127]. When cotton is treated with a dilute solution of hydrogen chloride in an aprotic solvent (e.g. benzene), the rate of hydrolysis is much more rapid than with aqueous acid of the same concentration. This is because the hydrogen Figure 1.20 Acid hydrolysis of mercerised and unmercerised cellulose at 20°C. (Source: [6,p.136].) 100 20 40 60 300 500 Time/h Fluidity/rhes Mercerised cellulose Unmercerised cellulose
  • 37. 30 CELLULOSE: STRUCTURE, PROPERTIES AND BEHAVIOUR IN THE DYEING PROCESS HomeContents chloride is abstracted from the solvent by the regain moisture in the cotton to give a highly concentrated aqueous solution of hydrochloric acid in close contact with the fibres [128]. For many years there was considerable controversy as to whether cellulose contained ‘weak bonds’, i.e. glycosidic bonds capable of being hydrolysed more rapidly than normal completely accessible 1,4-β-glucosidic linkages. Three possible causes of such acid sensitivity have been identified [21]: (1) The presence of electron–attracting groups at points in the chain molecules where they might ‘activate’ nearby glycosidic bonds. (2) The presence of occasional ‘rogue’ chain units (e.g. anhydroxylose units in place of anhydroglucose units). (3) The existence of a few glycosidic bonds that are more readily hydrolysable because they are under exceptional physical stress. The first possibility probably applies to wood pulp, which often contains a few aldehyde groups introduced during pulping, but it does not apply to cotton. The second possibility is unlikely to apply to any reasonably pure form of cellulose; the xylose and mannose occasionally found among the soluble hydrolysis products of cellulose probably arise from non-cellulosic minor components of the starting material. The third possibility has been firmly established as applicable to native cellulose, hydrolysis rate constants 104 times greater than normal having been observed in the very early stages of the reaction [21,129]. The present situation has been summarised elsewhere [30,p.227]. 1.5.3 Degradation by alkalis The glycosidic linkages in cellulose are not attacked by alkali at temperatures lower than about 170°C. However, the preparation of cellulosic textiles for dyeing frequently includes hot alkaline treatments that cause greater weight loss than can be accounted for by the removal of non-cellulosic substances. In the now obsolescent process of kier boiling, where temperatures as high as 140°C might be reached, weight losses of up to 4% might occur owing to dissolution of cellulosic fragments. Davidson [130] first recognised that these losses were caused by the stepwise removal of anhydroglucose units from the reducing ends of the cellulose chains, and Kenner and his collaborators later elucidated the mechanism [22,30,p.231]. They made a systematic study of the alkaline decomposition of O- substituted glucose derivatives, showing conclusively that Isbell’s mechanism for the formation of deoxyaldonic acids (formerly known as saccharinic acids) by the action of alkali on glucose could be applied to the alkyl ethers of sugars and to
  • 38. 31 HomeContents polysaccharides [131]. The work was extended later to hydrocelluloses, which differ chemically from cellulose solely in their content of reducing end groups. The first effect of cold dilute alkali on glucose is to convert some of it into mannose and fructose (Scheme 1.4). The formation of an equilibrium mixture of the three sugars was discovered by Lobry de Bruyn and Alberda van Ekenstein [132,133]. If the action of alkali is prolonged, more profound changes occur, leading to the production of three isomeric deoxyaldonic acids (IV, V and VI in Scheme 1.5) by elimination of the hydroxy group attached to the carbon atom next but one to that carrying the negatively charged oxygen in the intermediate ions I, II or III. The process has therefore been called ‘β-hydroxycarbonyl elimination’. It is immediately followed by a benzilic acid rearrangement (so- called because of the alkaline conversion of benzil to benzilic acid and designated here as BAR) of the resulting dicarbonyl compound. When the hydroxy group has previously been alkylated, elimination of the alkoxy group occurs by the same mechanism but more rapidly. It is then known as ‘β-alkoxycarbonyl elimination’. The mechanism of the terminal alkaline degradation of cellulose can now be formulated (Scheme 1.6). The reducing end group is first converted to a fructose residue which, having a carbonyl group in the β-position with respect to the glycosidic bond, becomes detached from the end of the chain and appears in solution as 3-deoxy-2-C-(hydroxymethyl)pentonic acids (isosaccharinic acids, V). At the same time a new end group is exposed and then removed by the same process. This erosion of end groups, often known as the ‘peeling reaction’, would DEGRADATION OF CELLULOSE Scheme 1.4 H CHO C OH C C HO H C H OH H OH CH2OH CH C OH C C HO H C H OH H OH CH2OH O– HO CHO C H C C HO H C H OH H OH CH2OH CHOH C O– C C HO H C H OH H OH CH2OH CH2OH C O C C HO H C H OH H OH CH2OH D-mannose H+ I OH– D-glucose H+ II D-fructose
  • 39. 32 CELLULOSE: STRUCTURE, PROPERTIES AND BEHAVIOUR IN THE DYEING PROCESS HomeContents CH C OH C C HO H C H OH H OH CH2OH O– CHO C OH CH C C H OH H OH CH2OH CHO C O CH2 C C H OH H OH CH2OH COOH CH(OH) C C C H OH H OH CH2OH H H CHOH C O– C C HO H C H OH H OH CH2OH CH2OH C O C C C H H OH CH2OH CH2OH C O C C C H H H OH CH2OH COOH C(OH)(CH2OH) C C H H H OH CH2OH OH O CH2OH C OH C C O– C H OH H OH CH2OH CH2 C OH C C C OH H OH CH2OH CH3 C O C C C H OH H OH CH2OH COOH C(OH)(CH3) C C H OH H OH CH2OH I IV O O H H2O II H2O V H2O III VI BAR BAR BAR BAR = benzilic acid rearrangement continue until all the material had dissolved unless some other process intervened to stop it. The fact that cellulose and hydrocelluloses show only limited weight loss on boiling with alkali means that some such ‘stopping reaction’ must occur. It has in fact been identified as β-hydroxycarbonyl elimination of OH from C3 in the reducing end group, converting it into a 3-deoxy-D-hexonic acid (metasaccharinic acid IV) residue still attached to the cellulose chain at C4 (VII). This end group is stable to alkali, which explains why native cellulose that has been purified by some form of alkaline treatment has acidic rather than reducing end groups. Scheme 1.6 represents only the main reactions of cellulose with dilute alkali. Even at temperatures encountered in textile processing some other reactions occur, and above 170°C there is also considerable random chain scission [30,p.237]. Scheme 1.5
  • 40. 33 HomeContents Scheme 1.6 CHO C OH C C HO H C H O(G)n H OH CH2OH CH2OH C O C C C H O(G)n H OH CH2OH CH2OH C O C C C H H H OH CH2OH CHO C C C C H O(G)n H OH CH2OH HO H CHO C O C C H H C H O(G)n H OH CH2OH COOH C(OH)(CH2OH) C C CH2OH H OH H COOH CH(OH) C C H C H O(G)n H OH CH2OH H HO H O H OH Erosion + H H Stabilisation V Further degradation VII DEGRADATION OF CELLULOSE 1.5.4 Degradation by enzymes The enzymic degradation of cellulose forms an essential part of the carbon cycle by which life on Earth is sustained. It is also one of the means by which, possibly in the not too distant future, cellulose may be exploited as a renewable source of industrial chemical feedstocks [35,134,135]. Biodeterioration of textiles is a serious problem and much effort has gone into methods of prevention. If damp cotton is exposed to air, mildew may gradually develop on it, accompanied by staining that is difficult or impossible to remove. Prolonged exposure also causes a serious loss of tensile strength, but little depolymerisation occurs. The tendering of cellulosic textiles by moulds and bacteria is thus easily distinguishable from that caused by acids even though both are due to the hydrolysis of glycosidic linkages [8]. The difference arises because the micro-organisms act on localised sites that are rendered soluble in water, leaving most of the cellulose unchanged.
  • 41. 34 CELLULOSE: STRUCTURE, PROPERTIES AND BEHAVIOUR IN THE DYEING PROCESS HomeContents Studies of degradation using cell-free enzyme preparations have proved difficult to interpret with certainty, although the mechanism of the process is now understood in broad outline [23,36,124,136]. Many cellulases extracted from fungi hydrolyse native cellulose only to a limited extent unless it has been rendered more accessible by some form of swelling treatment (e.g. mercerisation). Until recently certain extracts, notably those of Trichoderma viride and Trichoderma koningii, were thought to contain an ‘activating’ enzyme that rendered the cellulose molecules more accessible to the hydrolysing enzymes. The concept of a specific decrystallising enzyme has now been replaced by one of synergism between the component enzymes of the cellulase complex. Unlike the undigested residue of fungal attack, cellulose that has been immersed in an active cellulase extract has a reduced DP, but for a given weight loss the DP reduction is less than would have been caused by acid hydrolysis [136]. Thus enzymolysis does not occur randomly. It is not confined to the ends of chains, however, but it does occur more readily there, removing soluble oligosaccharide fragments. 1.5.5 Oxidative degradation Since each intermediate unit in a cellulose molecule contains three hydroxy groups, the number of possible oxidation products is considerable. End groups can also be oxidised and this may be important if primary oxidation is accompanied by chain scission, as often happens when the reaction occurs under alkaline conditions. Moreover, only a small proportion of a cellulose fibre is readily accessible to most oxidising agents, which therefore react rapidly at first and then very much more slowly. The action of most oxidising agents is non- specific and complex, and it is frequently impossible either to elucidate its mechanism or to determine precisely the constitution of the oxycelluloses produced. Nevertheless, a wealth of useful information has been obtained by the method of measuring the properties of series of oxycelluloses obtained with a variety of oxidising agents (section 1.5.1). When this method was extended to a study of periodate oxidation, where both the mechanism of the reaction and the constitution of the oxycelluloses have been satisfactorily elucidated, relationships between properties and constitution were recognised that could be used to explain the behaviour of other oxycelluloses by analogy. Only a brief account of the oxidation of cellulose will be given here as a recent comprehensive review is available [30,p.243]. Some of the most important early work was done with sodium hypochlorite [137,138] when this was the main bleaching agent for cotton, because it was
  • 42. 35 HomeContents necessary to find the cause of the excessive tendering that sometimes used to occur. The work led directly to the practice of adding sodium carbonate to bleach liquors, maintaining a pH of 10–11 to minimise the formation of reducing groups. These impair both the stability of bleached cotton on storage and its resistance to damage during laundering. The rate of oxidation was found to be at a maximum at pH 7, and later work [139] showed that the ratio COOH:CHO:CO was 2:7:9 at pH 5, but 5:1:0 at pH 10. Hydrogen peroxide is now the preferred bleaching agent for cotton. It is used in aqueous sodium hydroxide containing sodium silicate and stabilised against spontaneous decomposition catalysed by metal ions such as Fe(II) or Mn(II) by the addition of small quantities of Mg ions. As a bleaching agent it is generally regarded as ‘safe’, but of course if proper conditions are not maintained it can damage the cloth. Chemically, the oxidation of cellulose by alkaline peroxide and that by oxygen in the presence of alkali are similar. Carboxy groups and a few reducing groups are produced, and some depolymerisation occurs. The reaction is catalysed by transition metal ions and almost certainly involves peroxide radicals [140–142]. The oxidation of cellulose by atmospheric oxygen constitutes the process of ‘ageing’ to which wood pulp must be subjected to reduce its DP to a level suitable for the production of viscose (section 1.2.3). The simultaneous production of carboxy groups explains why viscose fibres contain more of these groups than purified cotton. Another safe bleaching agent for cellulose is acidified sodium chlorite (NaClO2). Only in highly concentrated solution does it cause significant oxidation, and then the active species is chlorine dioxide formed by the decomposition of chlorous acid [143]. Chlorite bleaching is declining in favour of peroxide-based methods, probably for commercial reasons. A curious phenomenon is observed when a cellulosic fabric is suspended vertically with its lower end dipping in water. The fabric acts as a wick and, in the presence of air, a brown line develops at the height to which the water rises [144–146]. The presence of acidic and reducing groups may be detected by qualitative tests. If air is rigidly excluded no brown colour appears, but the acidic and reducing groups are still formed. Presumably, the reducing groups are formed by hydrolysis arising from increased acidity at the wet–dry boundary and then carboxy groups are formed from them in a Cannizzaro-type rearrangement. The brown colour may be due to atmospheric oxidation of the primary products. These explanations are speculative, but the effect itself sometimes has unfortunate practical consequences. For example, brown lines have been found in window curtains that have been wetted by rain from time to time over several years [147]. DEGRADATION OF CELLULOSE
  • 43. 36 CELLULOSE: STRUCTURE, PROPERTIES AND BEHAVIOUR IN THE DYEING PROCESS HomeContents The oxidation of cellulose by sodium metaperiodate (NaIO4) is of particular interest because it adheres closely to the mechanism that operates with all vicinal diols and it is not confined to the readily accessible regions of the fibres; once they have been oxidised, the reagent slowly penetrates the entire crystalline regions [148,149]. Thus oxycelluloses with a constitution VIII may be prepared (Scheme 1.7). O OH OH CH2OH O OHC CHO O CH2OH O + IO4 + IO3 + H2O VIII Scheme 1.7 HOOC COOH O CH2OH O OHC CHO O CH2OH O HOH2C CH2OH O CH2OH NaBH4 O HClO2 Scheme 1.8 One of the most notable features of these oxycelluloses is their extreme susceptibility to degradation by alkali, which arises from their containing a β- alkoxycarbonyl group (section 1.5.3). When the aldehyde groups are oxidised with chlorous acid to carboxyl groups [150] or reduced with sodium borohydride to primary alcohol groups [151], as shown in Scheme 1.8, stability to alkali is restored [30,p.250]. Thus the periodate oxycelluloses provide an extreme example of latent tendering (section 1.8.2). Even though little chain scission occurs during oxidation, periodate oxycelluloses have very high fluidities in cuam because of the alkalinity of the solvent. However, when their aldehyde groups are either oxidised or reduced, their fluidities fall. They never revert completely to the fluidity of the original cellulose, probably because of degradative side reactions during the periodate oxidation. The tensile strength of cotton is reduced by periodate oxidation because of the formation of hemiacetal crosslinks between aldehyde and hydroxy groups in adjacent chain molecules (section 1.6.5) [152].
  • 44. 37 HomeContents DEGRADATION OF CELLULOSE Another reagent that can penetrate cellulose crystallites is dinitrogen tetroxide (N2O4). Although its main effect is to produce uronic acid groups (carboxy groups at C6) it also produces both ketone and aldehyde groups [30,p.253]. Materials containing about 2% by weight of carboxy groups have been found to clot blood and then to dissolve in it slowly. They have therefore been produced as absorbable haemostatic dressings for certain post-operative conditions where other methods of arresting the flow of blood are not appropriate [153]. 1.5.6 Degradation by heat Cellulose can be heated for many hours up to about 120°C without any serious deleterious effect. In dry air at higher temperatures, however, considerable depolymerisation takes place, accompanied by the formation of carbonyl and carboxy groups in the solid material, the evolution of water, carbon monoxide, and carbon dioxide, and a loss of tensile strength [34,154,155]. The formation of aldehyde groups in cellulose renders it inherently unstable. Cotton that has been overbleached with hypochlorite under acid or neutral conditions goes yellow on storage, even in the dark at room temperature. Viscose fibres always contain a small number of reducing groups arising from the ageing of the alkali cellulose during manufacture. These groups can cause yellowing problems in the dyeing of pastel shades by a continuous thermofixation process. Periodate oxycelluloses, which have large numbers of aldehyde groups, go yellow or brown rapidly on heating to 100°C, but prior reduction of these groups with borohydride inhibits yellowing. The yellow substance can be extracted with boiling 5% sodium bicarbonate. The visible and u.v. absorbance of the extract can be correlated with the aldehyde content of the material and the yellow substance has been tentatively assigned the structure of a β-diketone [156,157]. At about 250°C pyrolysis to numerous products becomes significant, and these have been extensively studied over many years because of the fire hazard associated with cellulosic textiles [34,155,158]. The most important solid product is laevoglucosan IX (Scheme 1.9) and among the volatile products are water, hydrogen, methane and other hydrocarbons, carbon monoxide, carbon dioxide and glyoxal. In a much simplified picture, pyrolysis may be represented as occurring by two paths, namely dehydration and laevoglucosan formation (Scheme 1.10). Dehydration, which is slightly endothermic, probably leads to the formation of intra- and intermolecular ether linkages, the latter constituting a form of crosslinking (X in Scheme 1.9). The formation of unsaturated rings tautomeric with 2-keto-3-deoxyanhydroglucose units (XI in Scheme 1.9) may also occur. The complex mixture is sometimes referred to as ‘dehydrocellulose’; it
  • 45. 38 CELLULOSE: STRUCTURE, PROPERTIES AND BEHAVIOUR IN THE DYEING PROCESS HomeContents Scheme 1.10 200–280°C Slightly endothermic 280–340°CEndothermic Exothermic Dehydrocellulose Char + gaseous productsCellulose Tar (mainly laevoglucosan) O HO OH OH CH2 O O OH O CH2OH O O OH OH CH2 O O OH CH2OH O O O CH2OH X IX O XIScheme 1.9 decomposes further exothermically yielding gaseous products and a mass of carbon known as ‘char’. Many of the gaseous products are inflammable and the char glows in the presence of air, being slowly converted to carbon dioxide. The other pyrolysis pathway results in the formation of tar, the main constituent of which is laevoglucosan. This reaction is strongly endothermic and occurs to a greater extent than dehydration at higher temperatures. It is evident that the search for flame retardants for cellulose must be for reagents that will increase the amount of laevoglucosan formed at the expense of dehydration. 1.5.7 Degradation by radiation In practice, the most important type of photochemical degradation that can affect cellulose is that caused by visible and near-u.v. radiation. This must, of necessity, be a photosensitised reaction since the radiation itself is not sufficiently energetic to disrupt the molecule. The energy needed to cleave C–C or C–O bonds in cellulose is about 330–380 kJ mol–1, from which it can be calculated that, to be effective, radiation must have a wavelength less than about 340 nm
  • 46. 39 HomeContents [24]. Since the visible region of the spectrum is roughly between 400 and 800 nm, it is not surprising that pure cellulose is scarcely affected by daylight. However, in the presence of oxygen and a photosensitiser considerable degradation occurs and this is enhanced by the presence of moisture. The oxycellulose formed is of the alkali-sensitive type and can be particularly objectionable in curtains or other domestic textiles. Although light exposure alone lowers the tensile strength very little, serious loss may occur on subsequent laundering. Incidentally, this is why it is often recommended that curtains should be dry cleaned. The most important group of photosensitisers includes the yellow, orange and red vat dyes, but sulphur and basic dyes in the same colour range and certain metallic oxides, such as those of zinc and titanium, are also active. Their action is complex and has been extensively discussed [24,37]. Many years ago, Egerton [9,159–161] showed that undyed cotton threads placed as far as 8 mm away from dyed threads exposed to sunlight in a moist atmosphere were themselves degraded (Figure 1.21). Egerton concluded that some form of ‘activated’ oxygen DEGRADATION OF CELLULOSE Figure 1.21 Photodegradation of cotton yarn sensitised by Cibanone Orange R (SCl CI Vat Orange 21) by sunlight in air at 100% r.h. (Source: [9].) 5 10 20 40 60 Incident light energy at 460 nm/mW h cm–2 × 103 Lossintensilestrength/% Dyed cotton Adjacent undyed cotton Isolated undyed cotton
  • 47. 40 CELLULOSE: STRUCTURE, PROPERTIES AND BEHAVIOUR IN THE DYEING PROCESS HomeContents must be involved and that either this species itself or hydroxyl radicals derived from its reaction with water caused the degradation. He later [162] attributed degradation to singlet oxygen, but this has been strongly disputed [37]. The direct photolysis of cellulose is a simpler reaction than photodegradation, but it is only brought about by u.v. radiation of shorter wavelength than is encountered by textiles in normal use. For research purposes, a quartz mercury- vapour lamp giving monochromatic radiation of wavelength 253.7 nm is frequently used. In contrast to photosensitised degradation, direct photolysis is unaffected by the presence of oxygen and is inhibited by vat dyes and by water. An interesting feature is that this degradation may continue in the dark for several weeks after irradiation has ceased, at a rate that increases with storage temperature. Some mention must be made of the effect of ‘high-energy radiation’ (also called ‘ionising radiation’) on cellulose. These terms usually refer to X-rays or γ- rays, but they also include β-rays and other high-speed electrons, α-rays and neutrons. Much work has been done with γ-radiation from cobalt-60. One effect is the formation of free radicals that are remarkably stable in the presence of moisture. Radicals have been detected by electron spin resonance (e.s.r.) spectroscopy as long as seven years after exposure of cotton to γ-rays. Since the spectrum consists of a triplet with a signal strength ratio of approximately 1:2:1, the radicals may have structures XII, XIII or XIV (Scheme 1.11), but confirmatory evidence is not available [25,37]. The e.s.r. spectrum of cellulose that has been irradiated dry consists of an intense but short-lived single line which is more easily explained as arising from the species XV or XVI (Scheme 1.11). These would be expected to lead to depolymerisation; this has been observed, together with the formation of O OH OH CH2OH O O OH OH CH2O O O OH OH CH2 O O OH OH CH2OH O O OH OH CH2OH XIV XV O XVI XIIIXII Scheme 1.11
  • 48. 41 HomeContents carbonyl and carboxy groups, in γ-irradiated cellulose. Small quantities of carbon monoxide and carbon dioxide are also produced. 1.6 DERIVATIVES 1.6.1 Introduction Any macromolecular product of the reaction of cellulose with another substance may properly be described as a derivative. However, the term is usually confined to esters, ethers and similar products. The chemistry of the esters and ethers of cellulose is now outlined, including graft copolymers and crosslinked derivatives, which often fall into one or other of these classes. The intermediate chain unit of cellulose contains three hydroxy groups capable of forming derivatives. The degree of substitution (DS) of a derivative is the fraction of hydroxy groups that have reacted, and can therefore have any value between 0 and 3. It does not define a derivative uniquely, however, because the pattern of substitution is seldom uniform. Spurlin [10] has defined the highest possible degree of uniformity as that resulting from a reaction in which every unit in the cellulose chain is equally accessible to the reagent. This definition takes no account of the differing reactivities of the three hydroxy groups. It is generally accepted that the primary alcohol group at C6 is usually much more reactive than the secondary groups at C2 and C3, but the relative rates differ from one reaction to another. Thus the reactivity ratio of primary to secondary alcohol groups is much higher for acylation with the bulky tosyl (p- toluenesulphonyl) group than with the small acetyl group. The overall relative reactivities of the three hydroxy groups under heterogeneous conditions are determined by the combined effects of three factors: their inherent chemical reactivity, steric effects arising from the size of the entering group, and steric effects arising from the supramolecular structure of the cellulose. In most reactions the reactivity decreases in the order C6 ≥ C2 ≥ C3. As a consequence of these differences, many different derivatives having the same average DS may exist. They may differ in distribution of substituents both within a single chain unit and along the length of the chain molecules. The properties of derivatives, especially their solubilities and their mechanical properties in the solid state, depend strongly on the distribution of substituents and on the DP, which is often reduced by degradation during derivative formation. Some depolymerisation is desirable in most commercial products, but its extent needs to be carefully controlled. Although interest in cellulose derivatives lies mainly in their industrial DERIVATIVES
  • 49. 42 CELLULOSE: STRUCTURE, PROPERTIES AND BEHAVIOUR IN THE DYEING PROCESS HomeContents applications, it must not be forgotten that they have played an important role in the establishment of the chemical structure of cellulose [30,82]. Direct acid hydrolysis is unsatisfactory for structural studies, partly because the glucose produced is often subject to further change under the conditions employed. Instead, derivatives (particularly methylcellulose) have been prepared under non- degradative conditions and subjected to either hydrolysis or methanolysis. The products of the complete hydrolysis of trimethylcellulose are 2,3,6-tri-O-methyl- D-glucose and one molecule of 2,3,4,6-tetra-O-methyl-D-glucose per non- reducing end group. Methanolysis gives the corresponding methylglucosides. 1.6.2 Esters [11,12,26,38,163,164] Cellulose can be esterified with most inorganic and organic acids by methods analogous to those used for simple alcohols. Many of the products have practical applications, but the most important have been the acetates, nitrates and xanthates. Cellulose nitrate is generally manufactured to a DS between 2 and 3 (11.11–14.14% N). The dinitrate esters have been used for plastics and the trinitrates for explosives; those in between are suitable for lacquers and similar products. Good solvents for dinitrated esters (11–12% N) include methanol, various alkyl acetates, methyl ethyl ketone and acetone, but at a higher DS only acetone is suitable. Although there are several ways of nitrating cellulose, aqueous mixtures of nitric and sulphuric acids are always used industrially. Here the equilibria illustrated in Scheme 1.12 are established. Nitration is effected by the nitronium ion (Scheme 1.13) and is a reversible reaction. Hence the DS achieved is governed by the composition of the nitrating bath and particularly by its water content. A typical bath composition (by weight) for a dinitrate would be HNO3 21%, H2SO4 61.5%, H2O 17.5%. The reaction is rapid (Figure 1.22), equilibrium being reached in about 10 min at a moderate temperature (20–40°C) 3H3O+ + NO3 – + 2HSO4 – HNO3 + 2H2SO4 + 3H2O NO2 + + H3O+ + 2HSO4 – + 3H2O Scheme 1.12 Scheme 1.13 Rcell OH + NO2 + + H2O Rcell ONO2 + H3O+
  • 50. 43 HomeContents DERIVATIVES Figure 1.22 Rate of nitration of cellulose in a mixture of nitric acid (21%), sulphuric acid (61.5%) and water (17.5%). (Source: [11].) 20 40 60 2 6 10 14 Time/min Nitrogencontent/% to minimise depolymerisation. This unusual rapidity occurs because the nitronium ion readily penetrates the crystalline as well as the amorphous regions of the fibres and reacts rapidly with the hydroxy groups. It is thus a homogeneous process, formerly described as a ‘permutoid’ reaction. The initial product contains a few sulphate ester groups. These are removed by boiling with dilute acid, neutralising with sodium carbonate, and washing well with water. If this were not done, the sulphate groups would gradually hydrolyse to sulphuric acid which would catalyse the decomposition of the cellulose nitrate, under some circumstances with sufficient violence to cause an explosion. Sometimes a stabiliser (e.g. a very weak base) is added to cellulose nitrate before storage. Cellulose acetates have a wide variety of uses, including conversion into textile fibres, transparent sheeting and moulded plastics. The dyeing of acetate and triacetate fibres is described elsewhere [165]. Acetylation may be achieved using acetic anhydride in the presence of an acid catalyst. The reaction is represented formally by Scheme 1.14, but the mechanism is more complex, involving intermediate combination of the catalyst with the cellulose. Two separate processes are used: the fibrous process and the solution process. In the former, suitable only for triacetate production, a mixture of acetic anhydride and acetic acid is used in a medium that does not dissolve cellulose acetate, e.g. an aromatic hydrocarbon or carbon tetrachloride; the catalyst is frequently perchloric acid. In laboratory investigations a mixture of
  • 51. 44 CELLULOSE: STRUCTURE, PROPERTIES AND BEHAVIOUR IN THE DYEING PROCESS HomeContents Rcell OH + (CH3CO)2O Rcell OCOCH3 + CH3COOH Scheme 1.14 acetic anhydride and pyridine has sometimes been used to effect partial fibrous acetylation [166]. In the solution process the cellulose is treated with a mixture of acetic anhydride, acetic acid and sulphuric acid. The reaction is slow and heterogeneous. The outermost layers are acetylated first and dissolve in the acetic acid, exposing unchanged cellulose to attack and dissolution in its turn. The process continues until a solution of cellulose triacetate (the ‘primary’ acetate) in acetic acid is obtained. The reaction is exothermic and external cooling is necessary to prevent depolymerisation of the molecular chains. The product contains sulphate ester groups that must be removed by hydrolysis, water being added under carefully controlled conditions. The triacetate (DS 2.8–2.9) may then be precipitated out by the addition of more water. The so-called secondary acetate, sometimes described as ‘diacetate’, has a DS of about 2.3 and is produced by carefully controlled hydrolysis of the triacetate at 40°C. This product is also precipitated out by copious addition of water. Secondary cellulose acetate made in this way is soluble in acetone, whereas derivatives with the same DS made by direct acetylation are not. This is a good example of the influence of the pattern of substitution on the solubility of a cellulose derivative. Sodium cellulose xanthate [31] is the sodium salt of the O-ester of unsymmetrical dithiocarbonic acid. It is made by the action of carbon disulphide on alkali cellulose (Scheme 1.15). The ester itself is of little interest; its importance lies in its solubility in dilute sodium hydroxide. The solution is called viscose and is used for the manufacture of regenerated fibres. The DS is usually 0.5 or a little more, but for the production of polynosic fibres it may be nearly 1.0. In the regenerated-fibre industry the DS is often expressed as the γ-number, which is the number of xanthate groups per 100 anhydroglucose units (γ = 100 DS). The technology of viscose production is too complicated to be discussed here, but two important features may be mentioned. The first is that the alkali cellulose (i.e. cellulose impregnated with 18% sodium hydroxide) must be ‘aged’ OC SNa S Rcell OH + NaOH + CS2 Rcell + H2O Scheme 1.15
  • 52. 45 HomeContents in air at 25–30°C for at least a day, to reduce the DP to a suitable value. The second is that the viscose solution itself must be ‘ripened’ by storage at 15–25°C for 1–3 days, to achieve favourable conditions for regeneration. The most important chemical change during ripening is gradual decomposition of the xanthate. The degree of ripening required depends upon the particular final product. 1.6.3 Ethers [13,27,39] Highly substituted cellulose ethers may be prepared in the laboratory by any of the methods generally applicable to polysaccharides, provided that due account is taken of the difficulty of making the material completely accessible to reagents [119,vol.3,p.274]. The DS of most commercial cellulose ethers lies between 0.5 and 2, and is commonly around 1.5. These materials are usually made by treating alkali cellulose with an alkyl halide or other alkylating agent in an autoclave at a temperature low enough to avoid degradation. Methylcellulose is made with methyl chloride (Scheme 1.16). Sodium chloroacetate is used for carboxymethylcellulose (RcellOCH2COOH) and ethylene oxide for hydroxy- ethylcellulose (RcellOCH2CH2OH). The primary hydroxy group in the hydroxyethyl grouping is itself reactive and therefore gives rise to side chains of the type –(CH2CH2O)nCH2CH2OH where n is about 2 (although in theory it could be much larger). The material is therefore analogous to the graft copolymers of cellulose (section 1.6.4). The reaction of cellulose with activated olefinic compounds in the presence of alkali also gives ethers. Cyanoethylcellulose is prepared in this way using acrylonitrile (Scheme 1.17). Vinylsulphones (CH2=CHSO2R), or precursors that generate them in the presence of alkali, can also be used to give sulphonylethyl ethers. Most commercial cellulose ethers are soluble in water. Some, such as methylcellulose, are less soluble in hot water than in cold, but others, e.g. DERIVATIVES Rcell OH + CH2:CHCN Rcell OCH2CH2CN NaOH Scheme 1.17 Rcell OH + NaOH + CH3Cl Rcell OCH3 + NaCl + H2O Scheme 1.16
  • 53. 46 CELLULOSE: STRUCTURE, PROPERTIES AND BEHAVIOUR IN THE DYEING PROCESS HomeContents hydroxyethylcellulose, show normal solubility behaviour. Their numerous applications stem from their capacity to form viscous solutions and gels with water and to act as protective colloids. Their use in the food industry is widespread and carboxymethylcellulose is usually incorporated in domestic washing powders to prevent the redeposition of soil during laundering. The only commercially available ethers that are soluble in organic solvents are ethylcellulose and cyanoethylcellulose of DS 2.5 or higher. The former finds a limited use in lacquers and moulded plastics, and the latter, because of its high dielectric constant, is a useful insulator. It is sometimes used in the manufacture of electroluminescent articles. 1.6.4 Graft copolymers [167] The possibility of modifying cellulose by grafting other polymers on to it is obvious, but extensive research on grafting has proved less commercially important than might have been hoped. Methods of grafting have usually involved homopolymeric grafts. Thus if cellulose is impregnated with a suitable vinyl monomer in the presence of a radical initiator (e.g. benzoyl peroxide, hydrogen peroxide/iron(II) sulphate, sodium persulphate or ozone), a cellulosyl radical is formed to initiate polymerisation (Scheme 1.18). Particular monomers that have been extensively studied include acrylonitrile, acrylamide, acrylic acid, methacrylic acid, vinyl acetate and styrene. This list is by no means exhaustive. Rcell O + CH2:CHR Rcell OCH2CHR Scheme 1.18 Graft polymerisation may also be initiated by means of γ-rays. Cellulose is irradiated together with the monomer to be grafted or prior to treatment with it. In either case a suitable swelling agent must also be used if a significant amount of grafting is to be achieved. One difficulty associated with grafting of cellulosic textiles is that most methods of initiation employed also lead to degradation of one kind or another. Thus in practice a balance has to be struck between the improvement due to grafting and deterioration due to degradation. 1.6.5 Fundamentals of crosslinking Cellulose can be crosslinked by any reagent containing at least two functional
  • 54. 47 HomeContents groups capable of reacting with hydroxy groups. Modified celluloses containing other functional groups may also form crosslinks. For example, an aldehyde group may combine with an alcohol group in a neighbouring chain to form a hemiacetal, or two carboxy groups in different chains may crosslink by forming a salt with a bivalent cation. The mechanical and certain other properties of cellulosic fibres are profoundly affected by crosslinking, and these effects are reflected in the properties of assemblies of fibres in ways that depend on how a particular assembly is constructed. Thus the tensile properties of textile yarns and fabrics usually correspond directly with the properties of their constituent fibres because the main influence on yarn stability is interfibre friction. On the other hand the stability of paper, which is a random web of short fibres, is mainly due to hydrogen bonds between fibres. The stability of the fibres themselves, whether in a yarn, in paper or in any other assembly, is due to a combination of internal hydrogen bonding and van der Waals forces. The introduction of covalent crosslinks into a cellulosic fibre has two important effects: it reduces the ability of the chain molecules (a) to move laterally, and (b) to extend longitudinally under stress. Evidence for the first effect is found in lower water retention (section 1.7.1), reduced swelling (section 1.9.1) and insolubility in solvents such as cuam or FeTNa (section 1.8.2). Insolubility in one of these solvents is a much-used test for the presence of crosslinking in cellulosic materials. Evidence for the second effect is more subtle [168]. Consider an imaginary fibre with no lateral forces between chains. It would be mechanically weak because, under stress, chain molecules would slide past one another and only those long enough to span the gap between the jaws of the testing instrument would contribute to the tensile strength. The introduction of crosslinks would increase the proportion of chains spanning the jaw gap and hence increase the load required to break the fibre. After a certain point, however, the introduction of still more crosslinks would restrict the movement of adjacent chains to such an extent that the proportion of chains capable of resisting applied stress would decrease and the tensile strength would fall. This is because a chain molecule can resist stress only when it is fully extended. In a real cellulosic fibre there are in fact very many intermolecular hydrogen bonds which, in their influence on mechanical properties, behave in the same way as other crosslinks. Their most important difference from covalent crosslinks is that they can be destroyed by water. This explains the well-known fact that the strength of cotton is increased by wetting, whereas that of regular viscose is reduced. In cotton the density of hydrogen bonding is already greater DERIVATIVES
  • 55. 48 CELLULOSE: STRUCTURE, PROPERTIES AND BEHAVIOUR IN THE DYEING PROCESS HomeContents than that required to produce maximum strength. Hence, on wetting, some of the bonds are severed, lowering the crosslink density and raising the tensile strength. In viscose, however, the density of crosslinking is less than that needed for maximum strength. Hence the loss of some crosslinks causes a reduction in tensile strength. For the same reason, the dry strengths of the two materials are affected in different ways by the introduction of covalent crosslinks. For example, cotton oxidised by periodate (section 1.5.5) suffers a rapid loss of strength because the hemiacetal crosslinks formed reinforce the effect of the crosslinking hydrogen bonds already present [152,169]. With viscose, on the other hand, the introduction of hemiacetal crosslinks at first increases the tensile strength until a maximum is reached, after which further crosslinking produces a loss of strength in the same way as with cotton. A recent elegant investigation of inter-chain esterification in cotton by means of 1,2,3,4-butanetetracarboxylic acid has led to similar conclusions [170]. In this work the breaking twist angles of the fibres were determined as a measure of brittleness. As expected, crosslinking increased the brittleness of the fibres. The magnitude of the mechanical changes depends on the length, number and distribution of the crosslinks introduced. The effect of the introduction of hemiacetal crosslinks into paper is an immediate increase in wet strength because in this case the crosslinks are predominantly between the fibres [171,172]. 1.7 AFFINITY FOR WATER AND ORGANIC LIQUIDS 1.7.1 Water The hydrophilic nature of cellulose is of great industrial importance for several reasons: (1) Water is essential for many processes, e.g. dyeing and finishing, paper making. (2) Cellulose cannot easily be freed from water completely; the fibres normally contain adsorbed water in quantities determined by the relative humidity and temperature of the surrounding atmosphere. (3) The mechanical properties of cellulosic fibres vary with their moisture content. (4) The comfort of clothing made from cellulosic fibres is closely associated with moisture absorbency. A cursory look at its formula (Figure 1.12) might lead one to expect that cellulose would be soluble in water, as glucose is. In fact, of course, it is not. It is
  • 56. 49 HomeContents generally accepted that water cannot penetrate cellulose crystallites [173]. The intermolecular hydrogen bonds holding the crystallites together are no stronger than those between water and cellulosic hydroxy groups. However, the inflexibility of the chain molecules prevents these bonds from being broken successively and the chance of them all being broken simultaneously is negligible [174]. Thus the insolubility of cellulose in water is a direct consequence of the equatorial orientation of all the hexose substituents, including the glycosidic bonds. This structural feature distinguishes cellulose from all other polysaccharides. As already mentioned, alkylcellulose ethers of low DS are soluble in water. This is because the alkyl groups distort the conformation of the cellulose chains so that crystallisation is inhibited and the remaining hydroxy groups are capable of being solvated. Adsorption of water vapour Even in highly crystalline cellulosic fibres the crystallites are relatively small, as in the proposed crystalline fibrillar structure of cotton (Figure 1.4). Hence there are numerous free hydroxy groups in the crystallite surfaces and in whatever disordered regions exist. These are the hydroxy groups responsible for the adsorption of water vapour from the air. The moisture relations of cellulose have been extensively studied over many years and have recently been reviewed by Zeronian [40]. The main features of the sorption isotherms were established in the classical work of Urquhart and Williams, illustrated for cotton in Figure 1.23 [175]. Sorption is expressed as moisture regain, i.e. grams of water per 100 g of bone-dry cellulose. Under average conditions in the UK, say a relative humidity (r.h.) of 65% at 20°C, scoured cotton has a regain of around 7.5%, which corresponds to a moisture content of 7%. This is equivalent to about two water molecules for every three anhydroglucose units in the sample, and may seem surprising for a material that feels perfectly dry. Fully mercerised cotton and regular viscose have regains of about 11% and 13% respectively under similar conditions. Later work [176,177] has added greatly to our detailed knowledge of sorption isotherms over an extremely wide range of ambient conditions. The sigmoidal curves in Figure 1.23 may be conveniently divided into three sections: (1) Low humidity (0–10% r.h.) – the progressive formation of a monolayer which, when complete, has one molecule of water bonded to each accessible hydroxy group. (2) Medium humidity (10–50% r.h.) – the formation of a multilayer of progressively increasing thickness. AFFINITY FOR WATER AND ORGANIC LIQUIDS
  • 57. 50 CELLULOSE: STRUCTURE, PROPERTIES AND BEHAVIOUR IN THE DYEING PROCESS HomeContents Figure 1.23 Sorption isotherms of scoured cotton at 25°C: adsorption (lower curve) and desorption (upper curve). (Source: [7].) 20 40 60 80 100 6 12 18 24 R.h./% Moistureregain/% (3) High humidity (50–100% r.h.) – the further sorption of water, virtually in liquid form and gradually swelling the fibres so that more hydroxy groups become accessible. Under most conditions the amount of adsorption decreased with increasing temperature, as expected. However, at relative humidities above about 85% there are discrepancies between the published results. Urquhart and Williams [175] found increased adsorption when the temperature was raised above 50°C, whereas Jeffries [176] did not. The difference probably lies in the methods of preparing the samples for examination. Jeffries subjected his samples to a rigorous stabilisation treatment by taking them through repeated sorption– desorption cycles at 90°C before measuring the isotherms. Urquhart and Williams used samples carefully purified by standard methods. Both sets of results are consistent with the idea that the structure of cellulose changes when exposed to high temperatures and humidities unless it has first been stabilised. It should be noted, however, that Jeffries’ treatment caused some depolymerisation which may itself have contributed to stabilisation. A notable feature of all cellulose–water isotherms is that the amount of water sorbed at a given r.h. is greater when equilibrium is approached from 100% r.h.
  • 58. 51 HomeContents than when it is approached from 0% r.h. (Figure 1.23). This phenomenon is known as hysteresis. If the starting r.h. is somewhere between 0 and 100% then adsorption or desorption curves between the two extremes in Figure 1.23 are observed. This has the practical implication that the precise moisture content of a sample of cellulose depends on its previous history. The cause of hysteresis has been the subject of much speculation. Urquhart’s attractively simple molecular explanation [178], that the number of hydroxy groups available to water at a given r.h. value is greater when the water has been desorbed than when it has been adsorbed, is now generally regarded as inadequate. It is difficult to reconcile with present views of fibre fine structure and with the multilayer theory of sorption [40]. In an alternative approach, Barkas [179] considered that internal stress is generated when cellulose adsorbs water from the dry state but when this stress is removed by subsequent gradual desorption, mechanical recovery of the fine structure is incomplete (because the material is plastic) so that more water is retained than had previously been adsorbed at the same r.h. value. Water imbibition Cellulosic fibres imbibe considerably more water when immersed in the liquid than they sorb at 100% r.h. [180]. The water can be held in several ways; in particular, a distinction is often drawn between ‘free’ and ‘bound’ water for the liquid within cell walls [40]. Outside the cell walls almost all the water is held by condensation in capillary voids within the fibrillar structure of the fibres. Much research on this subject has been carried out, but in practice the most useful method of measuring water imbibition is to weigh the amount retained after the sample has been centrifuged under standard conditions, e.g. for 30 min at 900 g. Obviously the result depends to a small extent on the conditions chosen. Cotton yarn gives a water imbibition of about 45–50%, and regular viscose about 90%, by this method. Greater retention is likely in the wet processing of fabrics, since excess liquor may be removed by less vigorous mechanical means, e.g. mangling. 1.7.2 Organic liquids Organic liquids other than aliphatic amines (section 1.9.3) are less strongly attracted to cellulose than water [14]. Small molecules with at least some hydrogen-bonding capacity, e.g. methanol or ethanol, are adsorbed from the vapour phase but to a smaller extent than water. As the size of molecules increases, the adsorption decreases, butan-1-ol showing no adsorption. Non- polar vapours such as benzene or carbon tetrachloride are sorbed only very slightly. AFFINITY FOR WATER AND ORGANIC LIQUIDS
  • 59. 52 CELLULOSE: STRUCTURE, PROPERTIES AND BEHAVIOUR IN THE DYEING PROCESS HomeContents Although only small quantities of non-polar liquids are imbibed by cellulose under normal conditions, they can be introduced into the structure by first swelling the material in water and then treating it successively with a series of solvents of decreasing polarity. After removal of the excess of the final solvent by pumping, the cellulose still contains an appreciable amount of the chosen non- polar solvent. Further pumping does not remove any of this residual solvent even when a high vacuum is used. Products of this type have been called ‘inclusion celluloses’ because their solvent content is not held by ordinary valency forces but is trapped in the cellulose structure in the manner of a clathrate compound. Apparently the originally swollen structure gradually collapses during the removal of the solvent by pumping. The significance of these compounds [28,181] is their higher reactivity in processes such as acetylation. They are stable only in the absence of water, which rapidly replaces the non-polar liquid. For example, if cyclohexane-inclusion cellulose is viewed under a microscope while being wetted, beads of liquid cyclohexane can be seen forming on individual fibres [181]. 1.8 SOLVENTS FOR CELLULOSE 1.8.1 Introduction For many years the only known solvents for cellulose were concentrated solutions of certain salts such as calcium thiocyanate at the boil, moderately concentrated mineral acids, e.g. 70% sulphuric acid, and cuprammonium hydroxide (cuam, Schweizer’s reagent), the latter two at room temperature [182]. Only cuam causes no depolymerisation, but to ensure this atmospheric oxygen and light must be rigidly excluded. Much effort has been expended since Schweizer’s time on the development of non-degradative solvents for cellulose [41,66]. Solvents for cellulose are required for two main purposes: (1) For testing, e.g. for assessing properties that are dependent on DP, or for detecting crosslinking, since a solvent that normally dissolves cellulose will not dissolve crosslinked material. (2) For making solutions from which cellulosic fibres can be regenerated. The first successful solvents were mostly similar to cuprammonium hydroxide, but with ammonia replaced by an aliphatic diamine (usually ethylenediamine) and copper replaced by some other metal. Much more recently a wide range of non-aqueous solvents has been developed. These two classes of solvent will now be discussed briefly.
  • 60. 53 HomeContents 1.8.2 Metal-complex solvents Over 60 years elapsed between Schweizer’s original discovery and Traube’s introduction of cupriethylenediamine as an alternative solvent for cellulose [183]. Both reagents are aqueous solutions of Cu(II) complexes. Cuprammonium hydroxide (cuam) is roughly 0.25 M in copper and has a Cu:NH3 molar ratio of about 1:50. On the other hand, cupriethylenediamine (cuen) is usually 0.5 M in Cu(II) with a Cu:NH2CH2CH2NH2 molar ratio of 1:2. Cuen is the more stable of the two solvents, mainly because the amine is practically non-volatile whereas ammonia is easily lost and the cuen complex is less sensitive to light. The copper complex in cuam is formed as shown in Scheme 1.19 [184], and that in cuen is formed in an analogous way. A series of solvents based on complexes of other metals with ethylenediamine was investigated by Jayme and coworkers during the 1950s [29]. The metals included cobalt, nickel, zinc and cadmium, the last-named being the most important in practical applications. The cadmium ethylenediamine solvent is approximately 0.45 M in cadmium and 0.35 M in sodium hydroxide with a Cd:NH2CH2CH2NH2 molar ratio of about 1:10; it thus contains considerably more ethylenediamine than cuen. It has the great advantage of being colourless and is usually known as cadoxen. SOLVENTS FOR CELLULOSE Jayme and Verburg [185] also discovered a solvent for cellulose in which Fe(III) is complexed with tartrate ions. The solution is green in colour and must be 2–3 M in sodium hydroxide. It is generally designated FeTNa, except in German literature where the symbol EWNN is used. EWNN stands for Eisen- Weinsäure-Natrium, the second N indicating that the sodium is added in two stages, first as sodium hydrogen tartrate and secondly as sodium hydroxide. The first industrial application of cuam was for the production of cupro; this has all but ceased. It has been used since the 1920s, particularly in the UK, for assessing chemical tendering in cotton and regenerated cellulosic fibres. In the British test [186], the flow time of a 0.5% (cotton) or 2% (regular viscose) solution in cuam is measured at 20°C in a special viscometer designed to exclude air. Careful temperature control is essential. The test result is expressed as the fluidity, i.e. the reciprocal of the dynamic viscosity; the unit of fluidity is the Scheme 1.19 O Rcell O Cu NH3 NH3OH Rcell OH + [Cu(NH3)4]2+(OH–)2 + 2NH4OH
  • 61. 54 CELLULOSE: STRUCTURE, PROPERTIES AND BEHAVIOUR IN THE DYEING PROCESS HomeContents reciprocal poise, or rhe (1 rhe = 10 m2 N–1 s–1 in SI units). The conditions of the test ensure an approximately linear relation between fluidity and loss of tensile strength. In this context, fluidity values are more meaningful, since viscosity falls roughly hyperbolically with loss of strength and is an insensitive measure of tendering except for slight decreases in strength [187]. Another important feature of the fluidity measurement arises from the high alkalinity of cuam. This means that it measures any ‘latent’ tendering present in a material, i.e. loss of tensile strength that becomes apparent in practice only after some form of alkaline treatment, such as laundering (sections 1.5.5 and 1.5.7). Many workers, particularly in North America, prefer cuen to cuam for assessing tendering and quality control. The TAPPI method [188] expresses the result as a viscosity, which is appropriate for its intended field of application (the paper and pulp industry). The ASTM method [189] expresses the result as intrinsic viscosity ([η]), from which the relative molecular mass M can be calculated if the constants (a and k) in the Mark–Houwink equation are known (Eqn 1.4): [η] = kMa (1.4) Many values of these constants have been published but frequently they do not agree with one another. Any of the solvents described may be used to test for crosslinking, but cadoxen and FeTNa are probably the most convenient. The behaviour of a fibre when wetted with a drop of solvent can be viewed under a microscope. Fibres that have no covalent crosslinks will dissolve fairly quickly; crosslinked fibres usually swell and sometimes burst, but they remain insoluble indefinitely except in rare cases where the solvent destroys the crosslink, e.g. by the hydrolysis of an ester. 1.8.3 Non-aqueous solvents Some non-aqueous solvents for cellulose have been known for some time, e.g. liquid ammonia containing certain inorganic salts [190] and mixtures of dinitrogen tetroxide with a variety of organic liquids [191]. Only within the last 20 years, however, has the growing awareness of environmental considerations aroused interest in these media. One incentive has been the search for a less objectionable solution than viscose from which to regenerate fibres. A significant start has been made, but viscose is not likely to lose its leading position for a long time. Although the systems under consideration are described as non-aqueous, the presence of some water is often essential for a good solvent.
  • 62. 55 HomeContents The most extensively studied system based on liquid ammonia is that in which the major constituent is ammonium thiocyanate [192,193]. A typical composition (by weight) would be 72.1% ammonium thiocyanate, 26.5% ammonia and 1.4% water. An interesting feature of solutions of cellulose in this mixture is that the ammonia can be displaced by various chemically disparate substances, e.g. γ-butyrolactone or a 1:1 mixture of morpholine and pyridine, without precipitation of the cellulose. The displacing substance must be miscible with the original solution; after its addition, ammonia is removed by pumping. An analogous solvent is dimethylacetamide containing 10% of lithium chloride; this gives highly stable solutions of cellulose. Fibres may be regenerated from these solutions by wet spinning in water or acetonitrile. These fibres have satisfactory physical properties [194], but the process has not yet proved commercially viable. The solvent power of certain cyclic tertiary amine oxides was reported in a series of patents from 1969 onwards [195,196]. Structural criteria for ability to dissolve cellulose were established and the crucial importance of having the right amount of water present was recognised [41]. The use of N- methylmorpholine-N-oxide (Figure 1.24) in the production of Tencel has already been described (section 1.2.3). It was also in 1969 that Schweiger proposed the use of N,N-dimethylformamide (DMF, HCON(CH3)2) containing around 15% of dinitrogen tetroxide as a cellulose solvent. It appears to act by forming cellulose nitrite which then dissolves (Scheme 1.20) [197,198]. Nitrosyl chloride behaves similarly and N,N-dimethylacetamide may be used instead of DMF. Satisfactory fibres may be regenerated by spinning into aqueous DMF containing certain salts [67,199], but commercial exploitation is not yet viable. The N2O4–DMF system differs from the others so far considered in that it dissolves cellulose by forming a soluble derivative. Another system in which a soluble derivative is formed is a mixture of dimethylsulphoxide (DMSO, (CH3)2SO) and paraformaldehyde (PF, (CH2O)n), [41,68,200–203]. In this case the derivative seems to be a hemiacetal formed between formaldehyde and the SOLVENTS FOR CELLULOSE Figure 1.24 N-Methylmorph- oline-N-oxide. (Source: [41].) O N O CH3 RcellOH + N2O4 DMF RcellONO + HNO3 DMF Solution Scheme 1.20
  • 63. 56 CELLULOSE: STRUCTURE, PROPERTIES AND BEHAVIOUR IN THE DYEING PROCESS HomeContents primary alcohol groups in the cellulose (Scheme 1.21). The solution is very stable, probably because of strong solvation of the methylol groups by the highly polar DMSO. Fibres have been spun from these solutions using aqueous solutions of formaldehyde scavengers such as ammonia, various amines or sodium sulphide. For cellulose pulps with DP between 400 and 600 a solution of convenient viscosity is obtained with a composition of cellulose 6%, PF 6% and DMSO 88%. Although fibres of good quality were obtained, the process has not yet been developed on an industrial scale. A minor, but important, use for DMSO–PF solutions of cellulose is in the preparation of unusual derivatives under homogeneous conditions [204]. For example, treatment with a mixture of methyl iodide and sodium hydride gives a mixed ether-acetal (Scheme 1.22). Some of the methylol groups are removed by sodium hydride so that the cellulosic alcohol group can be methylated; others are methylated whilst still attached to the cellulose, a stable full acetal being formed. It may be noted that solutions of cellulose in N2O4–DMF may also be used for the preparation of derivatives [197,198]. Scheme 1.22 O O HO OH O OHO O OH O OH O OH O O HO OH O OHO O O O O OCH3 CH3 NaH CH3I DMSOOCH3 RcellOH + HCHO DMSO RcellOCH2OH DMSO Solution Scheme 1.21
  • 64. 57 HomeContents 1.9 SWELLING 1.9.1 Introduction The aqueous swelling of cellulose has already been noted. When bone-dry fibres are immersed in water their average diameter may increase by as much as 20% [205]. In practice, however, cellulose is seldom encountered in a bone-dry state and it is usual to take the water-swollen state as the basis for swelling measurements, thus restricting the term swelling to increases in size greater than those that can be effected by water alone. Degrees of swelling are expressed as percentage increases in either diameter or cross-sectional area. Swelling always entails the breaking of hydrogen bonds between adjacent chain molecules. With water and dilute aqueous solutions this normally occurs only in accessible regions of the fibres, which often means between fibrillar surfaces. Penetration of the crystallites does not occur and the degree of swelling is always small. Some reagents, however, can penetrate the crystallites. This intrafibrillar swelling is always on a much larger scale than interfibrillar swelling. The phenomenon was discovered by Mercer in 1844 [206,207] using such reagents as 25–30% sodium hydroxide or 62% sulphuric acid at room temperature, or 59% zinc chloride at 70°C. Cotton yarn treated in this way swells in diameter and shrinks in length, so that fabrics become denser. The process also enhances dyeability. About 40 years later Lowe [208] discovered that if cloth was stretched to its original dimensions during subsequent washing, or if it was held under tension to prevent it from shrinking during treatment, it acquired a much improved lustre and smoothness. This process became known as mercerisation and is still widely practised, almost entirely with concentrated sodium hydroxide solutions [209,210]. It is advisable to confine this term to treatments with caustic alkalis and not to extend it to swelling agents such as liquid ammonia, since the two types of reagent are quite distinct in their action (section 1.9.3). Obviously swelling only partially disrupts the solid structure of cellulose; complete disruption would be equivalent to dissolution. A useful general picture of the process is illustrated in Figure 1.25 [211]. The swelling agent, or ‘foreign molecule’, breaks the hydrogen bonds in the crystallites, but has no effect on the van der Waals bonds which act at right angles to the hydrogen bonds. Thus the cellulose behaves as infinite sheets held intact by van der Waals forces and capable of being pushed apart by hydrogen-bond breakers to an extent determined by the size of the foreign molecule. SWELLING
  • 65. 58 CELLULOSE: STRUCTURE, PROPERTIES AND BEHAVIOUR IN THE DYEING PROCESS HomeContents Figure 1.25 Diagram of swelling of cellulose by penetration of reagent between sheets, looking along the fibre axis. (Source: J O Warwicker, in Liquid ammonia treatment of textiles, Shirley Institute, Manchester, 1970.) 1.9.2 Aqueous alkalis The literature on the swelling of cellulose by aqueous alkalis is extensive and sometimes confusing, but some excellent reviews are available [28,42,52]. The fundamental cause of swelling is the imbibition of water consequent upon the sorption of alkali by the cellulose. In one early study [212] of the sorption of sodium hydroxide, curves of the type illustrated in Figure 1.26 were measured by titration of the external solution. The main features were the two plateaux in the absorption curve, which suggested that definite stoichiometric compounds were being formed. It was subsequently shown, however, that the change in the titre of the sodium hydroxide solution represented the net effect of two separate processes, namely the sorption of sodium hydroxide and a preferential imbibition of water [213]. The water uptake passed through a maximum at a concentration of sodium hydroxide of 100–200 g l–1 (2.5–5 M), whereas the uptake of sodium hydroxide rose steadily in a manner characteristic of adsorption. These results were fully confirmed by later investigations, but that did not dispel belief in the formation Foreign molecule Direction of hydroxy groups Sheets of cellulose Cellulose chain a b
  • 66. 59 HomeContents SWELLING Figure 1.26 Apparent sorption of sodium hydroxide by cotton. (Source: [28].) of stoichiometric compounds by some workers for many years [214,215]. The main evidence for this theory came from the X-ray diagrams of ‘soda celluloses’ [214], but it is now generally agreed that these were incorrectly interpreted [28]. No air-dry sample of a soda cellulose with a composition corresponding to any of the proposed formulae has ever been reported. The degree of swelling of cotton in aqueous alkali depends on the amount of water imbibed, and this in turn depends on three factors: (1) The nature of the cation. (2) The concentration of the alkali. (3) The temperature. Early studies indicated that the essential feature distinguishing one cation from another was its degree of hydration under comparable conditions. Thus the maximum swelling at a given temperature for the series of alkali metal hydroxides was found to decrease in the order Li Na K Rb Cs, which is also the order of decreasing hydrating power [52]. Similar considerations apply to the effects of alkali concentration and temperature. Swelling, and the degree of hydration, decrease as the temperature is increased and pass through a maximum as the alkali concentration increases (Figure 1.27). The effect of temperature is also what would be expected from the fact that the sorption of alkali by cellulose 100 200 300 400 100 200 Concn of sodium hydroxide/g l–1 Absorption/gkg–1
  • 67. 60 CELLULOSE: STRUCTURE, PROPERTIES AND BEHAVIOUR IN THE DYEING PROCESS HomeContents Figure 1.27 Effect of temperature on swelling of cotton by sodium hydroxide. (Source: [28].) 4 8 1.0 1.5 2.0 Concn of sodium hydroxide/mol l–1 Degreeofswelling 0oC 25oC 100oC is exothermic [52]. The exceptionally sharp peak at 0°C and 3 M NaOH has been attributed to bursting of the primary wall of fibres [28]. The first quantitative theory of swelling was proposed by Neale, who applied the concept of a Donnan membrane equilibrium to the system as illustrated in Table 1.4 [216]. The membrane was the primary wall of the fibres and the non- diffusible ion the cellulosate ion RcellO–. The hydrogen ion concentration in the liquid phase was considered to be negligible. Donnan’s equation is Eqn 1.5: [Na + ]1 [OH – ]1 = [Na + ]2 [OH – ]2 (1.5) To achieve this, water must be driven from phase 2 into phase 1 by osmosis. The approximate osmotic pressure will be given by Eqn 1.6: P = RT([Na + ]1 + [OH – ]1 − [Na + ]2 − [OH – ]2 ) (1.6) Remembering that each phase is electrically neutral and neglecting concentration changes resulting solely from the increase in volume due to swelling, Eqns 1.5 and 1.6 may be used to calculate values of P and [NaOH]2 for arbitrarily chosen values of [NaOH]1. In Figure 1.28 the results so obtained are compared with measured values of the amount of water taken up by a regenerated cellulosic film (used instead of cotton fibres because it was easier to handle experimentally).
  • 68. 61 HomeContents SWELLING Figure 1.28 Swelling of cellulose film by sodium hydroxide: osmotic pressure (calculated); water absorption (measured). (Source: [6,Part 2,p.840].) The similarity between the two curves confirms in broad outline the premises of Neale’s theory. It is important to appreciate its shortcomings, however, as indeed Neale himself did. The theory does not even suggest why different cations should behave differently, and concentrations have been used instead of activities in spite of the fact that in concentrated solutions the two may differ considerably. Table 1.4 Distribution of ions immersed in sodium hydroxide Cellulose phase (1) Membrane Liquid phase (2) Cellulose RcellOH Cellulosate ions RcellO– Hydrogen ions H+ Sodium ions Na+ Na+ Hydroxide ions OH– OH– Water H2O H2O 8 16 24 20 40 20 40 Water absorption Osmotic pressure Concn of sodium hydroxide/mol kg–1 water Amountofwateradsorbed/molmol–1anhydroglucoseunit Calculatedosmoticpressure/atm
  • 69. 62 CELLULOSE: STRUCTURE, PROPERTIES AND BEHAVIOUR IN THE DYEING PROCESS HomeContents Scheme 1.23 The importance of ionic hydration in swelling was emphasised by Chédin and Marsaudon [217]. They visualised the process as arising from replacement of some of the water of hydration by cellulosic hydroxy groups, as represented by Scheme 1.23. When a hydrated ion pair is adsorbed by the cellulose, three molecules of water are released and replaced by three hydroxy groups, which are unlikely to be located in the same anhydroglucose unit. The liberated water molecules occupy a greater volume than when they are associated with the ion pair, and so cause swelling [218]. Thus not only are the differences in behaviour of alkali metal hydroxides accounted for, but the theory is applicable to other aqueous systems where ionic hydration is extensive. The theory is not, of course, applicable to non-aqueous swelling agents such as DMF. 1.9.3 Liquid ammonia and amines When cellulose is treated with anhydrous liquid ammonia at a temperature near its boiling point (–33.4°C) a 1:1 complex with a monoclinic cell having a = 1.27, b = 1.075, c = 1.03 nm and γ = 133.5° is formed [42,219–222]. This complex is fairly stable; it loses ammonia by evaporation, slowly at room temperature but more rapidly on heating, to give cellulose III. These changes are illustrated schematically in Figure 1.29 [223], which shows that the distance between the 110 planes remains nearly constant, the distension of the lattice by the ammonia being almost entirely within these planes (compare Figures 1.14 and 1.25). If the ammonia is removed by washing with water instead of by evaporation the material reverts to cellulose I. The morphology and mechanical properties of the fibres are also affected differently by the two methods of removing ammonia. The conversion of cellulose I to cellulose III is seldom complete and may not be permanent. Complete conversion has been achieved, however, by using liquid ammonia at 140°C under a pressure of approximately 11 700 kPa (1700 lbf in–2). The product is stable to boiling water [224]. Figure 1.29 also illustrates the effect of ethylamine (b.p. 16.6°C) on the cellulose unit cell. Once again a 1:1 complex is formed, but with greater distension in the 110 planes to accommodate the larger molecule. Propylamine behaves similarly, but complexes with higher aliphatic amines and some diamines can only be obtained if the cellulose is first ‘primed’ with liquid ammonia or ethylamine [225–229]. Water does not inhibit the action of Na+OH–nH2O + Rcell(OH)3 Rcell(OH)3 Na+OH– (n–3)H2O + 3H2O
  • 70. 63 HomeContents SWELLING (a) (b) (c) (d) 0 0.1 1 nm 020 110 110 Figure 1.29 Diagram of conversion of (a) cellulose I to (b) ammonia–cellulose, (c) ethylamine– cellulose and (d) cellulose III. ethylamine provided that the amount present is less than that required to form the monohydrate (28.6% by weight). If more water than this is present no complex is formed [230]. When ethylamine is removed from its complex with native cellulose a material of lower crystallinity and higher chemical reactivity is obtained. The product has therefore been called ‘decrystallised cellulose’ [166]. It is important to realise that decrystallisation is only partial. If the ethylamine has been removed by rapid evaporation it still gives a good cellulose I X-ray diagram. If removal has been by slow evaporation or by washing with a solvent, cellulose III is usually formed [231]. Figure 1.30 illustrates the influence of the nature of the solvent used to remove ethylamine on the chemical reactivity, here measured as the rate of acetylation by acetic anhydride in pyridine at 25°C. Washing with water gives a material of higher reactivity than the original cellulose, but washing with pyridine is even more effective. The relevant difference between the two solvents
  • 71. 64 CELLULOSE: STRUCTURE, PROPERTIES AND BEHAVIOUR IN THE DYEING PROCESS HomeContents Figure 1.30 Effect of ethylamine on rate of acetylation of cotton at 25°C. (Source: [166].) 50 100 150 10 20 30 Time/h Acetylcontent/% 1 2 3 1 2 3 – No pretreatment – Ethylamine treated, water washed – Ethylamine treated, pyridine washed is that pyridine can form only one hydrogen bond per molecule whereas water can form two. Thus unlike pyridine, water molecules tend to link neighbouring chain molecules. It should be noted that the pyridine-washed product had been further washed with water and dried in the air before acetylation in order to standardise the conditions of treatment. Clearly, once the ethylamine has been washed out with pyridine, treatment with water has less effect on reducing reactivity than when it is used directly to remove the ethylamine. In spite of early promise, ethylamine treatment of cellulose has not been exploited commercially. Liquid ammonia, however, has certainly achieved limited success [42,209,232,233]. Two different methods of removing ammonia have been proposed. In the Prograde process (J P Coats) the ammonia is washed out with water. This was used in the 1970s instead of mercerisation for sewing threads, and to improve the handle and dyeing properties of knitgoods. In the Sanfor-Set process (so-called because the inventors, the Norwegian Textile Research Institute, appointed the Sanforized Company as their worldwide licensing agent), most of the ammonia is removed by evaporation and the final traces by means of dry steam. It is often known technically as the dry-steam process and can be applied to both woven and knitted fabrics. In contrast to mercerising, it improves the softness, flexibility and resilience of woven fabrics
  • 72. 65 HomeContents STRUCTURE OF COTTON IN RELATION TO DYEING and reduces the amount of crosslinking agent required to produce satisfactory easy-care properties. However, its effect on dyeing is variable. 1.10 STRUCTURE OF COTTON IN RELATION TO DYEING 1.10.1 Introduction The dyeing properties of a cotton fibre are determined by its external and internal structure. Considerable variations between different types of cotton have been found. This is illustrated in Table 1.5 which compares the colours obtained when nine different cottons were dyed separately under the same conditions with Chlorantine Fast Green 5BLL (CGY, CI Direct Green 27) [234]. The importance of uniform blending of fibres before spinning into yarn is clear. The most obvious macrostructural feature to affect the dyeing of a fibre is its specific surface area, which is inversely related to its fineness (usually expressed as linear density). It has been known for over 50 years that the finer the fibre, the greater is the rate of dyeing [235]. The fineness of different cottons shows considerable variation. The linear density of St Vincent Sea Island cotton is about Table 1.5 Influence of cotton type on colour after dyeing with CI Direct Green 27 [234] __________________________________________________________________ Colour co-ordinates _________________________ Staple Type length/mm x y Y ∆E __________________________________________________________________ Pakistan Dessai 15.9 0.2369 0.2984 9.85 0 Middling American 27.0 0.2399 0.2971 9.60 1.47 Ashmouni 34.9 0.2382 0.2985 11.15 2.08 Bengal 12.7 0.2360 0.2971 10.70 1.46 Malaki 38.1 0.2359 0.2966 12.95 4.93 El Paso 30.2 0.2377 0.2967 12.05 3.50 Karnak 33.3 0.2385 0.2967 11.20 2.24 Sudan Sakel 31.8 0.2381 0.2976 11.00 1.87 Tanguis 31.8 0.2353 0.2967 10.40. 1.05 __________________________________________________________________ x = X X + Y + Z y = Y X + Y + Z X, Y (lightness) and Z are tristimulus values; ∆E is the total colour difference computed on the ANLAB 40 system.
  • 73. 66 CELLULOSE: STRUCTURE, PROPERTIES AND BEHAVIOUR IN THE DYEING PROCESS HomeContents 1.05 µg cm–1; whereas that of the coarse Bengal variety is about 3.30 µg cm–1; most American and Egyptian cottons lie between these two extremes [50]. Although dyeing, or indeed any other reaction, cannot take place within the crystalline regions of the fibre, it is not confined to the external surface. The accessible internal surfaces include the voids between microfibrils and the space between elementary fibrils, as represented by regions B and C in Figure 1.3, provided that the dye molecules are not prevented from entering by their size. One measure of the extent of these voids is the water imbibition (section 1.7.1), but perhaps a more relevant parameter for dyeing is accessibility measurement (section 1.4.3). However, these measurements suffer from the defect that the reacting molecules are mostly much smaller than those of a dye. For this reason some investigators have used the uptake of an actual dye as a measure of internal surface area. For example, Rousselle and coworkers studied the progress of the development of ordinary cotton fibres in the boll in this way [236]. They measured the uptake of CI Direct Green 26 (relative molecular mass, r.m.m. = 1350) by fibres taken from closed bolls at various times after flowering, using fibres transferred to the dyebath in the waterlogged state as well as fibres that had been dried (Table 1.6). The dyeability of fibres was drastically reduced by drying. Dye uptake decreased for both never-dried and dried fibres as the growing time (boll age) increased. Fibres harvested when the boll had been allowed to open in the cotton field, i.e. normal fibres, had the lowest uptake of all. It might be expected that, because of their bilateral structure (section 1.2.1), cotton fibres would dye more rapidly in regions B and C than in region A (Figure 1.3), but this does not appear to have been investigated. It is clear, however, that Table 1.6 Uptake of CI Direct Green 26 by cotton fibres at various stages of development. ________________________________________________________________ Boll age/ Dye uptake/ State of fibre days after flowering mg g–1 ________________________________________________________________ Closed bolls, never dried fibres 35 37.3 42 25.1 49 25.0 Closed bolls, dried fibres 35 2.9 42 2.4 49 2.2 Field-opened bolls, dried fibres 1.6 ________________________________________________________________
  • 74. 67 HomeContents STRUCTURE OF COTTON IN RELATION TO DYEING even if such differences occur they produce no overall visual effect in the dyed material. As already mentioned, any sample of cotton is likely to contain at least some exceptionally thin-walled fibres, either ‘dead’ or ‘immature’ (section 1.2.1). These fibres lack a fully developed secondary wall and so the proportion of cellulose in the primary wall is greater than in normal fibres. The substantivity of many dyes for the primary wall is less than for the secondary, and so the dyed thin-walled fibres appear paler than the rest. Furthermore, even in those cases where there is no significant difference in dye uptake, thin-walled fibres still appear paler because the average path length of the illuminating light within them is less. If the thin-walled fibres are uniformly distributed in a yarn or fabric, differences in their dyeing properties are obscured. However, this is seldom the case. During ginning and carding they tend to form small knots of tangled fibres known as neps [62], which appear paler than the bulk of a woven fabric. The fabric may therefore appear speckled. Other types of nep are also found in cotton [237]. The most important of these are seed-coat neps which are really motes with tangled fibres attached. A mote is defined as an aborted seed [62]. Since its composition is different from that of cotton fibres, it tends to dye a different shade. 1.10.2 Effect of swelling Mercer’s observation that swelling cotton with sodium hydroxide increases its dyeability is generally attributed to an increase in the number of accessible sites within the fibres. It is now also known, however, that even when the amounts of dye absorbed by mercerised and unmercerised cotton are identical, the mercerised sample often appears deeper [209]. This is illustrated in Figure 1.31 showing the amounts of four reactive dyes that can be saved by prior mercerisation to achieve a specific depth of colour. Even with pale shades (0.5–2% dye on weight of fabric, o.w.f.) as much as 30% less dye may be required, whilst for deep shades the saving is 50–70%. Goldthwait [238] established that the difference in apparent depth between mercerised and unmercerised dyeings varies with the actual depth of dyeing (Figure 1.32). Thus the extra dye required to achieve a match is not proportional to the depth of dyeing. Goldfinger [239] attributes the effect of mercerising on apparent depth to changes in the internal scattering of the incident light. The conditions of mercerisation have a significant effect on dyeability [240]. In general, the higher the concentration of sodium hydroxide used, the greater the dye uptake (Table 1.7). Usually (but not always) mercerised fibres absorb
  • 75. 68 CELLULOSE: STRUCTURE, PROPERTIES AND BEHAVIOUR IN THE DYEING PROCESS HomeContents Figure 1.31 Saving of reactive dyes at equivalent visual depths on mercerised and unmercerised cotton. (Source: [209].) 4 8 12 20 40 60 Amount of dye/% o.w.f. (unmercerised) Savings/% 1 4 2 3 1 2 3 4 – Procion Yellow H-E4R (ICI, CI Reactive Yellow 84) – Procion Blue H-EG (ICI, CI Reactive Blue 187) – Levafix Navy Blue E-4RA – (BAY, CI Reactive Violet 23) – Levafix Black E-B (BAY, – CI Reactive Black 5) Figure 1.32 Excess of CI Direct Blue 78 required by unmercerised cotton to match a mercerised sample, plotted against dye uptake. (Source: [238].) 1.0 2.0 0.1 0.3 Amount of dye on mercerised cotton/% o.w.f. Excessdyetogivematchon unmercerisedcotton/%o.w.f. dyes better if they are not dried before dyeing (Table 1.8). Similarly, lack of tension during mercerising tends to enhance dyeability, but exceptions to this rule are known (Table 1.9) [241].
  • 76. 69 HomeContents STRUCTURE OF COTTON IN RELATION TO DYEING Table 1.7 Effect of concentration of mercerising liquor on absorption of CI Direct Red 2 by cotton _________________________________ Sodium hydroxide Dye absorption/ concentration/% mg g–1 _________________________________ 0 17.7 0.85 23.9 13.5 29.5 17.5 31.5 22.5 33.8 27.0 35.6 31.5 36.0 _________________________________ The effect of liquid ammonia treatment on the dyeing of cotton depends on the way the ammonia is removed. Aqueous washing gives a product almost as dyeable as mercerised cotton. As with mercerising, treated yarns and fabrics appear more deeply dyed than untreated material having the same amount of dye present [233]. Successful operation of the dry-steam process depends on the fabric’s containing less than 5% of ammonia on entering the steamer [233]. The improvement in fabric properties has already been mentioned, but the dyeability is often only slightly greater than that of unmercerised cotton. Some dyes, however, are absorbed as strongly as by mercerised cotton. If more than 35% of Table 1.8 Effect on dye absorption of drying after mercerising ________________________________________ Absorption/mg g–1 ___________________ CI Direct CI Direct Red 2 Yellow 12 ________________________________________ Unmercerised 8 3 Mercerised and dyed without drying 25 10 Mercerised and dyed after drying in air 16 8 Mercerised and dyed after drying at 110°C for 1 h 13 5 ________________________________________ Table 1.9 Effect on dye absorption of tension during mercerisation ___________________________________________________________ Absorption/mg g–1 _______________________________ CI Direct Red 2 CI Direct Blue 1 ___________________________________________________________ Unmercerised 15 1.5 Mercerised with tension 29 2.7 Mercerised without tension 35 2.4 ___________________________________________________________
  • 77. 70 CELLULOSE: STRUCTURE, PROPERTIES AND BEHAVIOUR IN THE DYEING PROCESS HomeContents ammonia remains in the fabric entering the steamer, the product is no different from that obtained by washing with water; between 5 and 35% ammonia, intermediate properties result. 1.11 STRUCTURE OF REGENERATED CELLULOSIC FIBRES IN RELATION TO DYEING Since the differences in fine structure between different regenerated cellulosic fibre types are much greater than between one cotton and another, precise relationships between structure and dyeability are difficult to formulate. The subject has been considered in greater detail than is possible here by Holme [75,77]. When considering dyeability it is important to distinguish between rate of dyeing and the final equilibrium, which may take a long time to reach. As with cotton, the finer the fibre, the more rapid the rate of dyeing [235,242]. Thus the time taken to reach equilibrium is greater for coarse than for fine fibres. Again as with cotton, at the same dye content there may be visual colour differences between fibres of different titre (linear density in tex) because of internal optical effects [243]. Hollow viscose fibres dye more quickly than regular viscose because of the much greater surface area of the internal channels, but the equilibrium uptake is essentially the same [70]. Although the dyeing characteristics of cotton and regenerated fibres that depend mainly on fibre fineness are similar, those dependent on fine structure show considerable differences [244]. For example, Giles [245] found that the light fastness ratings of several direct dyes (measured on an integral scale from 1 to 8 [246]) were higher on viscose than on cotton. This is illustrated in Figure 1.33. If the light fastness had been identical on the two fibres, all the points would lie on a straight line of unit slope through the origin. An important structural feature of all regenerated fibres, but especially of regular viscose, is the difference between skin and core. The skin is more crystalline than the more readily swollen core. However, the skin crystallites are smaller than those in the core, and they are better oriented. Thus dyeing of the core is likely to be rapid once the dye liquor has reached it. In the well-known staining technique of Sisson and Morehead [247] for distinguishing skin from core in fibre cross-sections under the microscope, the dye is indeed more strongly absorbed by the core from aqueous solution. However, when the fibres are dried and then immersed in aqueous dioxan, the dye is leached out from the core only, leaving the skin deeply stained. The direct influence of the DP of regenerated fibres on their dyeability is small [248]. The indirect influence, arising through the relationship of crystallinity and
  • 78. 71 HomeContents STRUCTURE OF REGENERATED CELLULOSIC FIBRES IN RELATION TO DYEING 2 4 6 2 4 6 Light fastness on cotton Lightfastnessonviscose Figure 1.33 Comparison of light fastness of direct dyes on cotton and viscose. (Source: [245].) orientation to DP, is greater. DP is, of course, only one among several factors that combine to determine these two properties. That the degree of crystallinity is important in determining the dyeability of regenerated fibres is shown by the effect of steaming the fibres prior to dyeing. This is illustrated in Figure 1.34 [249]. Steaming of both regular viscose and cupro causes an increase in crystallinity which in turn leads to a decrease in dye absorption. An equally important fibre parameter is orientation [250]. Increased orientation is accompanied by decreases in both equilibrium absorption and rate of dyeing; the latter is by far the more marked. This is probably due at least in part to an associated decrease in the volume swelling of the fibres [251]. This is illustrated in Figure 1.35, in which optical birefringence is taken as a measure of orientation, and dye absorption per unit surface area of the fibres in 15 minutes as a measure of rate of dyeing [250]. The variation of surface area with filament denier has been taken into account by multiplying the actual absorption of dye by the square root of the denier, since the surface area of a filament is inversely proportional to the square root of its denier [242]. At low birefringence values (0.015–0.025) the equilibrium dye uptake was found to decrease only slightly as the orientation increased, but the effect was greater at higher degrees of
  • 79. 72 CELLULOSE: STRUCTURE, PROPERTIES AND BEHAVIOUR IN THE DYEING PROCESS HomeContents 100 140 0.4 1.2 2.0 Temp. of steaming/oC Dyeabsorption/%o.w.f. 1 2 3 Untreated 1 2 3 – Viscose, 0% stretch, 20–25% water content – Viscose, 30% stretch, 20–25% water content – Cupro, 35% water content Figure 1.35 Effect of fibre orientation on uptake of CI Direct Blue 1 by viscose. (Source: [250].) Figure 1.34 Absorption of CI Direct Blue 1 by regenerated cellulose fibres steamed for 1 h (dyeing conditions: 0.5 g dye and 5 g NaCl per litre for 2 h at 80°C. (Source: [249].) 0.02 0.04 1 2 3 Birefringence (65% r.h.) 15mindyeabsorption×(fil.denier)0.5 13.9 10.4 6.9 5.2 Filament denier
  • 80. 73 HomeContents REFERENCES orientation. Thus, when the birefringence increased from 0.04 to 0.15, the equilibrium dye absorption increased from 10 to 14 µg g–1[252]. So far the examples chosen to illustrate the relations between fibre structure and dyeability have been taken from studies using regular viscose. However, the same principles apply to modal fibres, which include all the old high tenacity fibres, such as high wet modulus and polynosic fibres. The most important special feature of modal fibres is their fibrillar structure, which means that they behave more like cotton than regular viscose. They usually have a high degree of lateral order; that is to say, their fibrils are uniformly distributed across a section of the fibre. As a result both the rate of dyeing and the equilibrium dye absorption are lower for modal fibres than for regular viscose. The behaviour of Tencel is anomalous in that it seems to absorb more dye than either regular viscose or cotton under comparable conditions [81], but this, of course, is a great advantage from a practical point of view. 1.12 REFERENCES 1. E Ott and H G Tennent, in Cellulose and cellulose derivatives, 2nd Edn, Part 1, E Ott et al., Eds (New York: Wiley-Interscience, 1954) 1. 2. A Payen, Compt. Rend., 7 (1838) 1052, 1125. 3. C B Purves, in Cellulose and cellulose derivatives, 2nd Edn, Part 1, E Ott et al., Eds (New York: Wiley-Interscience, 1954) 29. 4. J F Kennedy and C A White, Bioactive carbohydrates (Chichester: Ellis Horwood, 1983) 149. 5. G O Phillips and H Inagaki, preface to Cellulose: structural and functional aspects (report of the proceedings of Cellucon-88, Japan), J F Kennedy et al., Eds (Chichester: Ellis Horwood, 1989). 6. E Ott, H M Spurlin and M W Grafflin, Eds, Cellulose and cellulose derivatives, 2nd Edn, Parts 1– 3 (New York: Wiley-Interscience, 1954). 7. J A Howsmon and W A Sisson, in Cellulose and cellulose derivatives, 2nd Edn, Part 1, E Ott et al., Eds (New York: Wiley-Interscience, 1954) 251. 8. R G H Siu, in Cellulose and cellulose derivatives, 2nd Edn, Part 1, E Ott et al., Eds (New York: Wiley-Interscience, 1954) 183. 9. L F McBurney, in Cellulose and cellulose derivatives, 2nd Edn, Part 1, E Ott et al., Eds (New York: Wiley-Interscience, 1954) 172. 10. H M Spurlin, in Cellulose and cellulose derivatives, 2nd Edn, Part 2, E Ott et al., Eds (New York: Wiley-Interscience, 1954) 674. 11. J Barsha, in Cellulose and cellulose derivatives, 2nd Edn, Part 2, E Ott et al., Eds (New York: Wiley-Interscience, 1954) 713. 12. C J Malm and G D Hiatt, in Cellulose and cellulose derivatives, 2nd Edn, Part 2, E Ott et al., Eds (New York: Wiley-Interscience, 1954) 763. 13. A B Savage, A E Young and A T Maasberg, in Cellulose and cellulose derivatives, 2nd Edn, Part 2, E Ott et al., Eds (New York: Wiley-Interscience, 1954) 882. 14. J A Howsmon, in Cellulose and cellulose derivatives, 2nd Edn, Part 1, E Ott et al., Eds (New York: Wiley-Interscience, 1954) 425.
  • 81. 74 CELLULOSE: STRUCTURE, PROPERTIES AND BEHAVIOUR IN THE DYEING PROCESS HomeContents 15. N M Bikales and L Segal, Eds, Cellulose and cellulose derivatives, Parts 4 and 5 (New York: Wiley-Interscience, 1971). 16. J W Schappel and G C Bockno, in Cellulose and cellulose derivatives, Part 5, N M Bikales and L Segal, Eds (New York: Wiley-Interscience, 1971) 1115. 17. D W Jones, in Cellulose and cellulose derivatives, Part 4, N M Bikales and L Segal, Eds (New York: Wiley-Interscience, 1971) 117. 18. B A Tønnesen and Ø Ellefsen, in Cellulose and cellulose derivatives, Part 4, N M Bikales and L Segal, Eds (New York: Wiley-Interscience, 1971) 298. 19. Ø Ellefsen and B A Tønnesen, in Cellulose and cellulose derivatives, Part 4, N M Bikales and L Segal, Eds (New York: Wiley-Interscience, 1971) 158. 20. V W Tripp, in Cellulose and cellulose derivatives, Part 4, N M Bikales and L Segal, Eds (New York: Wiley-Interscience, 1971) 319. 21. A Sharples, in Cellulose and cellulose derivatives, Part 5, N M Bikales and L Segal, Eds (New York: Wiley-Interscience, 1971) 991. 22. G N Richards, in Cellulose and cellulose derivatives, Part 5, N M Bikales and L Segal, Eds (New York: Wiley-Interscience, 1971) 1007. 23. E T Reese and M Mandels, in Cellulose and cellulose derivatives, Part 5, N M Bikales and L Segal, Eds (New York: Wiley-Interscience, 1971) 1079. 24. P J Baugh and G O Phillips, in Cellulose and cellulose derivatives, Part 5, N M Bikales and L Segal, Eds (New York: Wiley-Interscience, 1971) 1047. 25. J C Arthur, in Cellulose and cellulose derivatives, Part 5, N M Bikales and L Segal, Eds (New York: Wiley-Interscience, 1971) 937. 26. G D Hiatt and W J Rebel, in Cellulose and cellulose derivatives, Part 5, N M Bikales and L Segal, Eds (New York: Wiley-Interscience, 1971) 741. 27. A B Savage, in Cellulose and cellulose derivatives, Part 5, N M Bikales and L Segal, Eds (New York: Wiley-Interscience, 1971) 785. 28. J O Warwicker, in Cellulose and cellulose derivatives, Part 4, N M Bikales and L Segal, Eds (New York: Wiley-Interscience, 1971) 325. 29. G Jayme, in Cellulose and cellulose derivatives, Part 4, N M Bikales and L Segal, Eds (New York: Wiley-Interscience, 1971) 381. 30. T P Nevell and S H Zeronian, Eds, Cellulose chemistry and its applications (Chichester: Ellis Horwood, 1985). 31. E E Treiber, in Cellulose chemistry and its applications, T P Nevell and S H Zeronian, Eds (Chichester: Ellis Horwood, 1985) 457, 469. 32. C H Haigler, in Cellulose chemistry and its applications, T P Nevell and S H Zeronian, Eds (Chichester: Ellis Horwood, 1985) 34. 33. A D French, in Cellulose chemistry and its applications, T P Nevell and S H Zeronian, Eds (Chichester: Ellis Horwood, 1985) 84. 34. S P Rowland and N R Bertoniere, in Cellulose chemistry and its applications, T P Nevell and S H Zeronian, Eds (Chichester: Ellis Horwood, 1985) chapter 4. 35. R Alén and E Sjöström, in Cellulose chemistry and its applications, T P Nevell and S H Zeronian, Eds (Chichester: Ellis Horwood, 1985) 531. 36. P Finch and J C Roberts, in Cellulose chemistry and its applications, T P Nevell and S H Zeronian, Eds (Chichester: Ellis Horwood, 1985) 312. 37. G O Phillips and J C Arthur, in Cellulose chemistry and its applications, T P Nevell and S H Zeronian, Eds (Chichester: Ellis Horwood, 1985) 290. 38. L C Wadsworth and D Daponte, in Cellulose chemistry and its applications, T P Nevell and S H Zeronian, Eds (Chichester: Ellis Horwood, 1985) 344.
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  • 88. 81 HomeContents 81 CHAPTER 2 Preparation William S Hickman 2.1 INTRODUCTION Cellulosic fibres, whether they are natural (cotton, linen, jute, ramie, sisal, etc.) or regenerated (viscose and polynosics) need some form of treatment to make them suitable for dyeing or finishing. This treatment, which removes natural or added impurities, is called preparation or pretreatment. It can be carried out on loose fibres, yarns or fabrics. Unless the fibre is uniform in whiteness, absorbency and chemical composition (the cellulose is not chemically modified) and has low levels of impurities (waxes, lignin, electrolytes, etc.), it is unlikely that it will take up dye or finish in a uniform way or to the maximum extent possible. The prepared fabric must have ‘fitness for purpose’. For example, a fabric to be given a water- repellent finish must be free from residual surfactant. Fibres used for medicinal end uses must have a low ash content. All such requirements must be met against economic constraints relating to the costs of water, labour, plant, chemicals and energy. The first, water, is particularly important. Not only can the purity of the water affect many of the requirements but the volume and the environmental impact of effluent can, and is, adding increasingly to the cost per kilogram of textile produced. The effluent with the highest biological/chemical oxygen demand (BOD/COD) stems from pretreatment. Successful preparation depends on four factors: (1) The level and type of impurities present. (2) The chemicals used in the various stages of preparation. (3) The water supply. (4) The type of machinery used. These will be discussed more fully after a brief historical review [1].
  • 89. 82 PREPARATION HomeContents The earliest methods of cotton preparation were based on boiling the fibre with potash liquor, extracted from the ashes of plants, followed by exposure to sunlight, if bleaching was required. Some of the terms still in use in the industry can be traced to such preparation. Thus a croft, which once was an open field where fabrics were exposed to light and air, is now the works where bleaching is carried out. Between alkaline boiling and crofting, fabric was rinsed and immersed in buttermilk, from which process is retained the term souring. In the late 1700s, lime replaced potash, sulphuric acid replaced buttermilk and the methods of bleaching were revolutionised as the observations on the bleaching action of chlorine, discovered by Scheele in 1774, were developed. In the 19th century subsequent developments were related mainly to improvements in scouring techniques using lime or caustic soda. Early in this century, a detailed study was made of hypochlorite bleaching, and methods, still used today, were developed to assess the properties of the bleached fabric. In the 1930s hydrogen peroxide and sodium chlorite bleaching processes were developed for cellulosic fibres. Hydrogen peroxide, however, had been used for fifty years prior to this to bleach wool and silk. Most developments since 1945 have been associated with these two chemicals. The most notable was the development of rapid, continuous bleaching processes with hydrogen peroxide. 2.2 IMPURITIES PRESENT IN CELLULOSIC FIBRES Most natural fibres are not pure cellulose but contain significant amounts of other materials [1,2]. The analysis of various cotton and flax samples is summarised in Table 2.1. The pectins are polygalacturonic acids and their Table 2.1 Impurities present in cellulosic fibres (% by weight) ___________________________________________________________________ Cotton [2] Flax [3] _____________________________ ________________ Average American Egyptian Unretted Retted ___________________________________________________________________ Cellulose 88.0–96.0 91.0 90.8 56.4 64.1 Hemicellulose 15.4 16.7 Pectins 0.7–1.2 0.53 0.68 2.5 1.8 Lignin 2.5 2.0 Proteins 1.1–1.9 Wax 0.4–1.0 0.35 0.42 1.3 1.5 Ash 0.7–1.6 0.12 0.25 ___________________________________________________________________
  • 90. 83 HomeContents calcium, magnesium and iron salts, and the ash contains calcium, magnesium and potassium phosphates and carbonates. Not included in these figures is the vegetable debris produced by mechanical harvesting and ginning of the cotton. This includes cotton seed, leaf and calyx for cotton. The amount depends on the quality of the cotton fibres. Linters (short staple waste) used for medicinal cotton can contain up to 25% of this woody material. In linen the vegetable debris is the residual cortex of the plant which has been left from the decorticisation process and can amount to nearly 30% of the weight of retted flax. Regenerated cellulosic fibres have much lower levels of impurities and these generally arise from the solvent used for dissolving the cellulose. To these natural impurities are added 10–15% of size, spin finish or knitting lubricant. The spin finishes and knitting lubricants contain surfactants and mineral oils applied alone or in admixture to decrease friction on machine parts. Sizes are film-forming materials used to coat the warp yarns of woven fabrics in order to minimise yarn breakage during fabric production. Starch, usually esterified or etherified, is the most common size but polyacrylates, cellulose ethers (e.g. carboxymethylcellulose), poly(vinyl alcohol) and poly(vinyl acetate) are also used. The source from which starch is obtained, e.g. corn or rice, can affect its ease of removal. Size plasticisers such as tallow or paraffin wax are either added to the sizing liquor or applied, after the size has been dried onto the yarn, by kiss–roll coating. Metallic ion contamination, particularly iron and copper, is of most concern during oxidative treatment as this can lead to chemical damage. This damage can manifest itself as very localised destruction of the fibre or reduced dye uptake. This form of contamination can result from the growing area where the plant absorbs the mineral matter through its roots or the wind simply blows the soil onto the fibre. It can also arise from washing off with contaminated water or heating with contaminated steam. 2.3 CHEMICALS FOR PREPARATION Many types of chemicals are used in preparation. These include enzymes (α− amylase, cellulase), alkalis (caustic soda, soda ash and phosphates), bleach bath stabilisers (silicates, organic stabilisers), sequestrants (amino- and hydroxy- polycarboxylic acids, aminomethylphosphonates, sodium polyphosphates), oxidative bleaching agents (hydrogen peroxide, sodium chlorite and hypochlorite) and reducing agents (sodium bisulphite, hydrosulphite). To these main components are added the auxiliaries, usually surfactants, to assist in CHEMICALS FOR PREPARATION
  • 91. 84 PREPARATION HomeContents wetting, scouring and detergency. The chemistry of these products has been covered elsewhere [4], but some guidance on the characteristics and selection of such materials will be given. 2.3.1 Enzymes Amylases degrade the components (amylose and amylopectin) of starch and starch-derived sizes very efficiently. Traditionally they are used warm (65–70°C) with a reaction time of several hours but thermostable enzymes have been developed which allow brief dwell times at temperatures up to 120°C. Common salt and calcium ions improve the degradation process. Copper and zinc ions and most anionic surfactants deactivate the enzyme. Amylases are discussed further in section 2.6.2. Cellulase is finding increasing use as a surface modifier for denim garments and as a handle modifier for certain regenerated cellulosic fabrics. As such processes are regarded as finishing processes, they will not be discussed further. There is some interest in the use of pectinases as scouring agents and lignases as lignin removers but as yet no commercial processes have been developed. Xylanases are used for pulp delignification. 2.3.2 Wetting agents and detergents [4] Good wetting is necessary for complete saturation and penetration of the substrate as quickly as possible. The rate of wetting is not usually so critical for batch processes but is vital to continuous processes. The exception is the processing of loose fibre where a deaerating agent is added to ensure complete, rapid penetration of the fibre mass. Commercial wetting agents are usually anionic or nonionic surfactants, or a blend of the two. The anionic products used by the textile industry are usually long-chain alkyl or aryl sulphates, sulphonates or phosphates. For most applications it is important that residues in the effluent should be biodegradable. To achieve this the alkyl groups must be linear rather than branched. In solution the compounds dissociate, to produce a hydrophobic tail (R in Scheme 2.1) with a hydrophilic polar head. It is this combination of hydrophobic and hydrophilic parts which makes the materials surface-active. Nonionic surfactants do not dissociate in solution but have a polyoxyethylene chain which Scheme 2.1 ROSO2ONa ROSO2O– + Na+
  • 92. 85 HomeContents is hydrophilic, readily forming hydrogen bonds with water molecules. Most commercial products used in textile preparation processes are ethoxylated fatty alcohols. Their general formula is shown in Figure 2.1. The surface activity is determined by the length of the alkyl group (R) and the degree of ethoxylation (n). Optimum surfactant properties are found when n = 10–15. If n is lower than this the product is more hydrophobic than hydrophilic and so has low water solubility and cloud point (the temperature at which the product precipitates from solution) and tends to have good fat-emulsifying rather than wetting properties. Though both anionic and nonionic surface-active agents can have good wetting and detergency properties, it is the ability of the nonionics to improve detergency (at the cost of reduced wetting performance) as the degree of ethoxylation is decreased that makes them useful. As stated above, a low degree of ethoxylation improves fat and wax removal but reduces cloud point. To maintain detergency as the temperature is raised the cloud point is often increased by addition of an anionic product [5]. The phosphate esters can be tailor-made similarly by altering the ratio of alcohol to phosphorus pentoxide during their preparation and are finding increasing use as detergents. The important questions about the selection and use of surfactants in preparation are the following: (1) If an anionic wetting agent is used in a desize liquor, is it compatible with the α-amylase? (2) Is the surfactant stable at the alkali and electrolyte concentrations used? This was of most importance in feed liquors for continuous wet-on-wet application of alkaline liquor (section 2.5.4) where the feed liquor can be five to ten times more concentrated than the application liquor, but is less critical now all chemicals are kept separate at this dosing stage. (3) Is the surfactant stable to oxidation? (4) Will the surfactant leave traces which would impair the result of subsequent processes, e.g. oil- and water-repellent finishing or printing? (5) Is the cloud point of any nonionic surfactant exceeded? During application this would adversely affect wetting/detergency whereas during washing-off it would precipitate the surfactant onto the substrate and cause backwetting. (6) Does the surfactant generate foam? In general, low-foaming behaviour is required and this can be temperature-dependent. In circulating-liquor CHEMICALS FOR PREPARATION R(OCH2CH2)nOH Figure 2.1 General formula of ethoxylated fatty alcohol.
  • 93. 86 PREPARATION HomeContents systems (jets, autoclaves), foam causes pump cavitation which leads to poor flow leading to unlevelness. On conveyor and roller-bed steamers the foam reduces traction. (7) Is the viscosity of the product, especially on dilution, satisfactory for use in automatic dosing systems? 2.3.3 Scouring agents [4] BASF pioneered the ‘demineralisation’ of cotton during scouring [6]. In this process a scouring auxiliary consisting of surfactant, a sequestering agent and a reducing agent removes iron or other metallic contamination (which can cause catalytic decomposition in subsequent bleaching) and promotes fabric absorbency by the extraction of the alkaline earth metals. The preferred surfactants were the alkylsulphates and the alkylarylsulphonates but the phosphate esters are increasingly used [7]. The reducing agent helps remove iron contamination by reducing ferric to ferrous salts, which are more readily complexed by the sequestrant. Furthermore, it has a reductive bleaching effect so that scoured cotton is less brown in colour than is usual in caustic-scoured material. Air in the fabric or in the steam causes tremendous degradation of the cellulose under hot alkaline conditions by the peeling reaction, or β-hydroxycarbonyl elimination. The mechanism will be discussed more fully later but consists of oxidation of the glucose units in such a way that a keto group is formed two atoms away from the glycoside link. This sensitises the link and causes the glucose unit to peel off. Reducing agents, for example the sulphites and hydrosulphites, are often added to scour liquors to prevent this auto-oxidation. Scouring is usually carried out with sodium hydroxide but the addition of other alkalis (trisodium phosphate, sodium silicates and tetrasodium pyrophosphate) and solvents has also been recommended [8]. The addition of oxidants to batch processes has been recommended but the greatest benefit is gained by adding oxidant to the scouring liquor on continuous ranges to give a so-called oxidative desize. 2.3.4 Sequestrants or chelating agents [2] Sequestrants, also called chelating agents, remove metal ions from solution. They do this by having adjacent groups (carboxylic or phosphonic acid, hydroxy, etc.) which form multidentate complexes with cations. The order in which the cations are sequestered depends on pH, but at a given pH this order will normally be the
  • 94. 87 HomeContents same for all sequestrants within a particular class (see below) and the relative stability of the complexes will also be the same. With a particular cation the stability of the complex and the amount bound will vary from sequestrant to sequestrant. The major classes of sequestrants are the aminopolycarboxylic acids (e.g. ethylenediaminetetraacetic acid, EDTA), hydroxypolycarboxylic acids (e.g. citric acid), aminomethylphosphonic acids (e.g. ethylenediaminetetramethyl- phosphonic acid, EDTMP), polycarboxylic acids (e.g. oxalic acid) and the sodium polyphosphates (e.g. sodium hexametaphosphate). The last two classes are not so important in preparation. Most sequestrants, excepting inorganic phosphates, sequester calcium over the whole pH range [8]. The sequestration of iron varies greatly but few products sequester iron above pH 12. 2.3.5 Stabilisers Bleaching with hydrogen peroxide is controlled by addition of a stabiliser. Commercial products are based on various combinations of magnesium salts, organic sequestrants, protein degradation products, surfactants and anionic polymers such as polysilicates and polyacrylates. Some products are designed to give stabilisation only, whereas others also combine detergent and softening actions. Many products give adequate stabilisation in batch bleaching but the selection for continuous steaming processes is much smaller. Control of chlorite bleaching is more concerned with control of pH and the emission of chlorine dioxide to the atmosphere. Hydrogen peroxide can even be used for this last purpose. 2.3.6 Oxidative bleaching agents Sodium hypochlorite, sodium chlorite and hydrogen peroxide are the three oxidants predominantly used for bleaching cellulosic fibres. Potassium permanganate is occasionally used for denim washing which confers a ‘stressed look’ to denim garments, and peracetic acid is being looked at as a replacement for sodium hypochlorite [9,10]. Detailed methods of use will be given later, but a brief overview of properties and problems is given here. Permanganate will not be discussed because of limited use and the environmental unacceptability of discharging manganese ions. Sodium hypochlorite Sodium hypochlorite (hypo or chemic) is produced by dissolving chlorine in CHEMICALS FOR PREPARATION
  • 95. 88 PREPARATION HomeContents caustic soda solution and is a yellow liquid with a specific gravity of about 1.2. The maximum commercial strength is 150 g l–1 of available chlorine. The solution is not very stable and so it is essential to give the chlorine content of the stock liquor a routine check. Hydrometry should not be used, as absorbed carbon dioxide rapidly changes specific gravity. Hypo bleaches rapidly at room temperature. It is not as sensitive to heavy metals as peroxide but it does easily chlorinate any organics present. This chlorination is an advantage in bast fibre processing but is a severe disadvantage in most other circumstances. For example, it yellows grey cotton by chlorinating the pectins and proteins present. It is essential, therefore, that the cellulose be cleared of natural impurities by thorough scouring before bleaching with hypo. In practice, one or two alkaline boils may be required with intermediate washing. These will be followed by scouring and further washing prior to bleaching. Bleaching must be followed by washing and an antichlor treatment. The end result is a prolonged, labour-intensive process with relatively low chemical costs but high demand for water. Hypochlorite bleaching today is largely a batch process but has found some use in India [11,12] and Eastern Europe on continuous bleach lines as a prebleach prior to peroxide. The chlorinated products from hypo are detected in effluent streams as adsorbable organohalogens (AOX) and chloroform. These are materials regarded increasingly as environmentally unacceptable. It is likely that hypo will disappear from use in the textile industry. Studies have shown that hydrogen peroxide does not form AOX compounds even in the presence of salt [13]. Sodium chlorite Sodium chlorite is usually sold as a solid (80% active solids) or as a liquid (26% active solids) with a specific gravity of 1.25. In contrast to hypo or peroxide bleaching, it is used under acidic conditions. The scouring process, required for hypochlorite, can be omitted and bleaching can be carried out on loomstate or desized materials. It bleaches seed well. Bleaching under acidic conditions removes only small amounts of natural fats and waxes. This was an advantage for knitting yarns and knitted fabrics as it gave a softer handle. Chlorite bleaching is relatively unaffected by iron or copper contamination. One of the bleaching agents liberated from chlorite under acidic conditions is chlorine dioxide. This corrosive gas is toxic (threshold limit value = 1 p.p.m.). Adequate fume extraction is essential, therefore, to ensure operative safety. The
  • 96. 89 HomeContents corrosive nature of chlorine dioxide demands exotic materials of construction such as titanium. One company used to supply special ceramic tile-lined equipment for chlorite bleaching [14]. Chlorite bleaching has little effect on oil stains but most dyes are destroyed by the process. For this reason fluorescent brightening agents are usually applied in the wash-off and few coloured-woven fabrics are bleached with this agent. It is frequently used for bleaching 100% polyester but the continuous, long-dwell J- box ranges used for bleaching knitted cotton fabrics have now been replaced by rapid, continuous peroxide bleaching. The AOX generated by chlorite is only a tenth of the amount generated by hypochlorite [13], but even this is eight to ten times that present after bleaching with hydrogen peroxide. Hydrogen peroxide Hydrogen peroxide is a colourless liquid which is used, in the textile industry, as a 35 or 50% solution with specific gravity of 1.131 or 1.195 respectively. It is the most widely used bleaching agent. Hydrogen peroxide is an extremely versatile bleaching agent, applicable over a very wide range of bleaching temperatures (ambient to 130°C) and times (minutes to days) on a wide range of machinery. Bleaching is usually carried out under alkaline conditions and this allows combination with other processes, such as scouring. Hydrogen peroxide can also be activated by agents other than the hydroxide ion, for example u.v. radiation and O- or N-acyl compounds. Irradiation generates free radicals, unsuitable for cellulose bleaching but of increasing interest in waste-water decolorisation [15,16]. The acyl compounds produce peracids, e.g. reaction of acetic anhydride (an O-acyl compound) with hydrogen peroxide produces peracetic acid, formula CH3COOOH. Peracetic acid is of increasing interest as a prebleach for knitted fabrics [9] and in the washing of denim and bleaching of cotton/acrylic blends [17]. In situ production has been recommended, but storage of the acetic anhydride seems more inconvenient than the commercially available peracid itself. 2.3.7 Reducing agents [3] Many coloured materials are decolorised by reducing agents such as the sulphites, bisulphites and hydrosulphites. The colourless form is sometimes insoluble in water and may be easily reoxidised to the coloured form. These reasons limit the use of reducing agents as bleaching agents. Sodium CHEMICALS FOR PREPARATION
  • 97. 90 PREPARATION HomeContents hydrosulphite (hydros) is used for the bleaching of wool but only in combination with hydrogen peroxide. Hydros is a powder that dissolves in water with difficulty and rapidly decomposes unless the solution is alkaline. Stabilised hydros has alkali added to the powder to improve solution stability. Reducing agents are used as antichlors (chemicals that remove the last traces of chlorine from the fibre after a hypochlorite or chlorite bleach). Their use in this area is diminishing, often on environmental grounds (odour, toxicity), in favour of hydrogen peroxide. 2.4 WATER [18] Most bleachworks were originally situated near a river which provided a copious supply of good quality water, i.e. water with low suspended solids, neutral reaction and low metallic ion content, particularly transition or heavy metal ions. The need for the following quality factors is self-evident: (1) Any suspended solids are likely to be filtered out onto fibre, yarn or fabric. (2) High or low pH could cause spontaneous, uncontrolled reactions to occur. (3) Metallic ions may give rise to discoloration, stains or chemical damage (catalytic tendering) of the substrate during bleaching. It is no longer possible to meet the original geographical requirements and water is now obtained as: (1) surface water from rivers, canals and reservoirs; (2) ground water from wells and bore holes; (3) mains water from the public supply; and as the quality can be variable it usual for the water to be: (1) filtered, using sand filters, to remove the suspended solids; (2) neutralised to give a pH around 7; (3) treated, if necessary, to remove residual chlorine; (4) softened to remove calcium and magnesium ions; (5) treated to precipitate heavy metal ions. During desizing, calcium ions are beneficial, whereas copper and zinc ions deactivate the enzyme; hence moderately hard water can be used but supply through copper piping should be avoided. In scouring, calcium ions, in the presence of hydroxyl ions, produce one of the most insoluble materials, calcium hydroxide, which deposits onto fabric and machinery. In bleaching, the precipitation of calcium hydroxide can seed
  • 98. 91 HomeContents stabilisers, causing them to coprecipitate. The heavy metals cause rapid peroxide decomposition and chemical damage. For peroxide bleaching some water hardness is advantageous. Ideally this should be about 60 mg l–1 (as calcium carbonate). With water of lower hardness an addition of magnesium salts is recommended. The precipitates from alkaline scouring or bleaching liquors cause an unlevel appearance and inadequate absorbency of the substrate. Such faults can usually be corrected by souring with acid. The bleacher is concerned not only with the quality of the water coming into the works but also with the quality of its discharge, whether this is to sewer or river. For discharge to natural waterways a consent limit for quality of effluent must generally be met. The parameters of concern to the preparation department are primarily volume of effluent, the BOD or other basis for assessing inorganic and organic chemical pollutant load, and suspended solids. Coloured effluent is becoming more of an issue. In some areas of the UK this now includes a specification of colour and this will obviously require increased cooperation between dyehouses and water treatment works. There is increasing interest in colour removal [13,19]. The efficiency of washing machines and a reduced number of washes, for example by combining stages, obviously reduce effluent volume but preparation is still the major user of water and so water reuse needs to be developed: this is likely to be an area for progress in the future. Correct selection of auxiliaries and breakdown of extracted material by oxidation, particularly starches, during processing can significantly reduce BOD/COD. Filtration, or at least settling-out in holding tanks before discharge, is increasingly being carried out to reduce suspended solids. Settlement is improved by the use of flocculants, e.g. aluminium sulphate, which will also flocculate some colouring and other organic matter. In the UK, the formation of the National Rivers Authority and the privatisation of the water authorities are making water supply and discharge increasingly expensive as tighter and tighter specifications are set. As the preparation department uses the largest amount of water, this is an area of increasing concern. 2.5 PREPARATION MACHINERY [20] The machinery used for preparation is, to a large extent, controlled by the physical form (loose stock, yarn or fabric) of the textile being processed and so this will be discussed first. In the sections dealing with actual machinery, brief comment only will be made as the book edited by Duckworth [20] and other reviews [21–23] deal with specific and often practical detail. PREPARATION MACHINERY
  • 99. 92 PREPARATION HomeContents 2.5.1 Form of textile Loose stock Loose stock is traditionally bleached in batch form on package machines or in autoclaves. Bales of fibre are mechanically opened, an essential cleaning operation, and packed into cakes. This packing is done wet with a stamping machine and the fibres are formed into packages on stainless steel rings or packed into a perforated basket. Packing densities of 250–300 g cm–3 are typical and limited by the strength of the container. Natural fibre in the form of short- staple waste (from ginning, carding or combing) is processed in this way. Viscose fibres have been continuously dyed for many years but the technology was never used for natural fibres [24]. During the 1970s, however, a continuous processing unit was developed for cotton in the USA by Cotton Inc. [25]. The unit forms the fibres into a sheet called a ‘batt’ and this batt is processed, with minimum tension, as if it were fabric. Yarn [26] Yarn is processed only for certain end uses, for example sewing thread, knitting yarns, viscose muffs. This represents only 15–20% of the material that is wet- processed. The processing can be carried out either in hank form or as packages. Continuous processing of hanks is only carried out for wool yarns. Packages are processed in either vertical or horizontal spindle machines. Such machines may be pressurised to allow wet processing to be carried out above 100°C whereas hank bleaching machines have horizontal supports and are usually operated at atmospheric pressure. Hank processing produces a yarn with a bulky, soft handle as it is free to move and swell in the liquor. This freedom of movement does, however, result in more tangling of the yarn which makes rewinding more difficult. Hank reeling and backwinding of the processed yarn are costly and wasteful. The voluminous nature of the yarn makes it difficult to pack the batch into the machine, which leads to levelness problems, and so batch sizes are smaller than in package machines for a given volume of liquor. Package processing, on the other hand, produces less bulky yarns and allows faster winding and backwinding with less fibre waste and bigger batch sizes per unit volume of liquor. These bigger batch sizes and the sophisticated process control found on package machines have now totally converted the processing of 100% cotton and polyester/cotton yarns to such machines. It is, therefore, cellulosic fibres and their blends, on package machines, which are primarily
  • 100. 93 HomeContents considered here, though similar bleach formulations can be used for such yarns in hank form. This process control encompasses production of standard packages in terms of weight, traverse, diameter and density, constant/invariable cycle times and press packing to even out any slight variability of winding. Uniform density is essential to prevent unlevelness caused by channelling. Knitted goods Knitted goods have poor dimensional stability and so are deformed easily by mechanical forces. They may be prone to creasing during wet processing. Knitted goods are comfortable to wear when made into garments, owing to their high elasticity and moisture absorbency. Preparation should not only give the required whiteness, absorbency and seed removal, but also a soft, voluminous handle and good sewability without distortion of the goods. A soft handle can be achieved by using acidic processing stages, which have no scouring action, to leave natural fats and waxes. The demand for supersoft finishes demanded application of softener. This change promoted the use of alkaline systems with the guarantee of good absorbency. Yarns used for knitting are usually high-quality combed yarns and so contain little non-cellulosic material. The knitted structure (fabric or hosiery) is open and promotes penetration of treatment liquors and facilitates washing-off of residual chemicals and impurities. Garments and hosiery are usually batch processed but the latter is being continuously processed in the USA. The batch machines are usually D- or Y- pocket washer extractors, though paddle machines are still used. This is a specialised area and will not be discussed further: however, the traditional bleaching agent for hosiery, hypochlorite, is being replaced by hydrogen peroxide on the environmental grounds discussed previously. Fabric can be prepared batchwise on winch or jet, continuously processed on J-box, spiral winch or jet (Argathen, Colorado, Jemco II) and conveyor steamer, or semi-continuously prepared by cold pad–batch methods. Woven fabrics Fabric construction and fibre content (100% cotton/linen/viscose or polyester/ cotton blends) determine whether fabrics are best processed in open-width (O) or rope (R) form. Table 2.2 summarises information from BASF but Evans also gives some ‘rules’ [27]. The more tightly woven a fabric, the more rigid it is. This PREPARATION MACHINERY
  • 101. 94 PREPARATION HomeContents rigidity and the demand for wider fabrics make rope processing less easy to carry out. In new installations, batchwise or continuous rope machines are seldom considered as they severely limit flexibility of processing. Machinery is available for continuous or batch operation for both forms. Rope processing is carried out on kiers, winches, jets and in J-boxes, whereas open-width processing is carried out on jig, beam, pad–steam ranges and open- width J-boxes. In some cases dyeing and finishing can be carried out on the same machinery but, more usually, the machines are specific to preparation. 2.5.2 Batch processing Kiers [28,29] Kiers came in many forms, the most popular being a cylindrical iron vessel placed vertically and holding two tons of fabric. Liquor percolates through the fabric and is pumped back, via a multitubular heater, to spray on the fabric. The machine is included here as historically it holds an important place in scouring and bleaching practice. In the Western world it has been superseded by other equipment. Garrett in a classic paper stated that for successful use of kiers it is essential to have all air excluded from the kier and: (1) the correct quantity of cloth in the kier; (2) the correct loading of the kier; Table 2.2 Fibre composition of various fabric constructions (O, open-width; R, rope form) ____________________________________________________ Polyester/ Construction Cotton cotton Viscose Linen ____________________________________________________ Sheetings O/R O/R Tablecloths O/R O Corduroy O O Furnishings O/R O O Interlinings O/R O O Pile fabrics O Ticking O/R O Shirtings O Tarpaulins/tenting O O Raised goods O O Twills O/R O ____________________________________________________
  • 102. 95 HomeContents PREPARATION MACHINERY Figure 2.2 Diagram of pack- age machine. (Source: [26].) (3) the correct rate of liquor circulation; (4) the correct liquor ratio; (5) the correct steam pressure and volume [29]. The fabric is rope washed after processing and, if bleaching is to follow scouring, the kier must be reloaded with intermediate washing. Iron vessels are unsuitable for peroxide bleaching and so the machine must be suitably passivated using silicate cement [28]. Package machines [20,26] Package machines are usually cylindrical vessels with a domed top and bottom which are capable of being pressurised (Figure 2.2). The top, fitted with a counter-balanced hinge, forms the lid and is locked into place by a sliding ring or similar device. Treatment is carried out by pumping liquor to the base of the vessel and, after it travels through the substrate, returning to the pump through another pipe located at the bottom of the vessel. Machine diameters go up to about 2 m with the vessel height being about the same but there are many tall units used for preparation. The machines can be supplied with a wide range of carriers for yarn or loose stock processing. Pressurisation can be achieved using an orifice plate (a plate with a small hole
  • 103. 96 PREPARATION HomeContents in its centre) which partially blocks the pipework to the side tank. The secondary pump attempts to deliver a greater amount of liquor to the vessel than is returned to the side tank and this creates a static pressure. Most machines are today pressurised by the air-cushion principle. Here the machine is not fully filled with liquor. When all valves are closed, the gas space above the liquor is compressed as the temperature rises. This gas space is usually fitted with control valves connected either to atmosphere or to a compressed gas supply and these valves maintain the desired pressure. This technique reduces the liquor ratio and so gives improved economy. All machines are fitted with pressure relief valves but it is still recommended that machines used for peroxide bleaching are fitted with bursting discs. Winch Winch design and use has been well reviewed by Wyles [20,22]. The basic arrangement is to have several (1–40) looped ropes of fabric of equal length which are kept apart by the peg rail. Most of the fabric is plaited down into the liquor but part of the rope runs over two reels above the liquor. The smaller diameter (15 cm) circular roller mounted about 35 cm above the liquor surface is called the fly or jockey roller. The upper reel, usually elliptical in shape, is motor-driven and moves the fabric through the liquor. Wyles deals with almost all aspects of winch operation including the use of the mid-feather (gate) to control fabric movement, the covering of the reel to improve traction and the use of drip feed buckets to add chemicals. Jet Developments in jet machines are well covered [20–22,30]. The main pump circulates the liquor which propels the fabric, in rope form, round the machine. The principle worked well for 100% synthetic fibre fabrics; however, the running speed and the high lift of the fabric from the liquor were unsuitable for preserving the appearance of knitted cotton fabrics. The Thies Ecosoft, one of many modern jets, has not only overcome these problems but also improved the economy of wet processing by reducing the liquor ratio, enabling the machine to process a whole gamut of fabric weights and styles from single jersey to fleece fabrics. This last point is exactly the same as the one made about open-width processing: only buy a machine if it is versatile. Such versatility is often aided by process control. An example of this is the Synchron Control of Thies, which
  • 104. 97 HomeContents adjusts pump speed and therefore rope speed to suit the particular stage of processing in an attempt to preserve fabric appearance [31]. Jig The jig or jigger is one of the oldest machines for processing woven fabrics in open-width form. Fabric is transferred from one roller to the other through a small volume of liquor (Figure 2.3). All aspects of the machine are well described elsewhere [20]. The tension involved in the winding operation makes the machine unsuitable for knitgoods. Particular problems of jig operation, as far as preparation is concerned, are the swelling and dissolution of size, which makes the fabric slippery and unstable in roll form, and the low liquor ratio which makes washing-off difficult. Beam-dyeing machines Beam-dyeing machines are, to some extent, analogous to horizontal package- dyeing machines. The carrier, or beam, is a perforated cylinder with an open end which locks on to the pump. The fabric is wound on to the beam and the beam is loaded on a cradle which runs on rails on a transporter. The beam is locked onto the pump by running the cradle from the rails on the transporter to rails inside the machine and screwing on a locking nut. The perforations on the beam that are not covered by the fabric are covered by flexible sheets of metal wrapped round the beam and held in place by large jubilee clips. The positioning of these sheaths is critical to successful beam-dyeing [20]. It is rare to beam-process cellulosic fabrics but gauze and batts of loose cotton are occasionally processed on these machines. 2.5.3 Semi-continuous processing Semi-continuous routes entail batching fabric on to A-frames, but an element of PREPARATION MACHINERY Figure 2.3 Diagram of jig dyeing machine. (Source: [20].)
  • 105. 98 PREPARATION HomeContents continuity is created by refilling the A-frames. Once the process has been started there is continuous flow of fabric through applicators, washers and dryers. Pad–roll Pad–roll processing involves application of chemicals to open-width woven fabric and rolling the fabric onto an A-frame contained in a movable container, often referred to as a caravan. Before batching, the fabric is warmed by passage through a pre-heater. Problems with pad–roll processing are twofold: (1) Variable treatment from first to last end. (2) Variability from edge to middle. The first, variation from one end of the batch to the other (ending), is simply caused by the time difference between batching the first and last end, which can be up to 2 h. The second is caused by swelling of the fibres in hot liquors. This swelling ‘squeezes’ the roll on the A-frame and transfers impurities and/or chemicals from the centre to the edge (listing) leading to unlevel results. Pad–batch Pad–batch processing is similar to that described above except that the batch is at room temperature. This reduces the ending and listing problems but increases the treatment time by a factor of 5–10. The long dwell times allow stages (desizing, scouring, bleaching) to be combined, resulting in substantial savings [33]. The result is never as good as that obtained from hot processes but it offers adequate quality with minimal capital expenditure. It is necessary to keep the batch wrapped in a plastic sheet to prevent it drying out. In both pad–roll and pad–batch procedures the roll must be kept turning to stop drainage occurring which would lead to irregular treatment and make the roll almost impossible to unwind. The technique was traditionally used on woven fabric but is being increasingly used for knitgoods, particularly in slit form. 2.5.4 Continuous processing [32,34,35] All continuous application systems require: (1) an accumulator (‘marking time’ machine); (2) application of chemicals;
  • 106. 99 HomeContents (3) storage of the impregnated substrate; (4) washing-off of the treated substrate. Accumulators are rarely discussed but are critical to continuous (i.e. non-stop) operation. The description not only covers powered accumulators but also devices such as scrays or dry J-boxes in which fabric can be stored either to facilitate the sewing together of batches of fabric or to compensate for speed differences in various sections of the machine. The application stage implies consumption of chemicals and so a system is usually also required to replenish them. Chemical application Consistent, uniform application of chemicals is a major contributor to successful preparation. This, irrespective of form, involves some form of saturator. Spray application has never been used in cellulosic fibre preparation but the Raco-Yet of Ramisch Kleinewefers does apply steam and chemicals together directly to the fabric [36]. Typical saturators are shown in Figure 2.4. The open-width saturator can be used to process woven and centre-slit knitted fabric, or in some instances fibre in sliver or batt form. The rollers and tension bars of such saturators are often essential components in promoting interchange and penetration of the application liquor. The rope saturator is used for both woven and knitted goods in rope form. The sieve-drum saturator can also be used for sliver and knitgoods, the latter in collapsed tubular form. Invariably there will be some form of pick-up control on exit from the saturator. PREPARATION MACHINERY Open-width saturator Rope saturator Figure 2.4 Various types of traditional saturator: (a) rope saturator; (b) open-width saturator. (Source: [34].)
  • 107. 100 PREPARATION HomeContents Traditionally this is a pair of squeeze rollers but the Flexnip [37,38], Optimax (Figure 2.5) [39] and Super-Sat [40] are new developments in saturator design which not only give increased control of the pick-up but add versatility to the saturator. Preparation invariably involves swelling of fibres and dissolution of impurities and so the higher the pick-up the better. Both the steam purge device and vacuum im- pregnation can increase this pick-up on heavyweight, dense fabrics [41]. The auxiliary suppliers also make liquor viscosity modifiers which enhance pick-up, for example, Irgapadol FE (S). Chemical addition is made to a side-tank, usually fitted with a lint filter and circulation pump to agitate and dilute the mixture as quickly as possible. In the Raco-Yet the mixture is supplied to the spray head. The liquor taken out of the saturator is referred to as pick-up, percentage pick-up or expression. It always refers to the percentage increase in weight of the dry fabric caused by the liquor. Storage devices [32,34,42] Most of these are steamers but for knitted fabrics they are winch- or jet-like devices through which the fabric moves spirally. The devices used to process wool or acrylics continuously, e.g. the microwave-heated Smith Fastran unit, are seldom used for cellulosic substrates. J-boxes The J-box was the first fully continuous preparation range for woven fabrics (Figure 2.6). Its basic design evolved, in the USA, from the Gantt piler, formerly used for hypochlorite bleaching [1]. This was a wooden structure shaped like the letter J with rolls at the bottom of the curve to facilitate movement of fabric. Two distinct types of rope J-box were developed for bleaching woven fabrics which differ mainly in the method of fabric heating. In the Becco, or open, J-box, heating is by direct application of steam, through manifolds at various heights, to the pack of plaited fabric. With the DuPont, or enclosed, J-box, the fabric is Figure 2.5 The Optimax saturator. (Source: [39].)
  • 108. 101 HomeContents PREPARATION MACHINERY Enclosed J box Open J box Figure 2.6 Diagrams of traditional J-Boxes: (a) enclosed J-box; (b) open J-box. (Source: [34].) heated by passing through a steam atmosphere in a heater tube adjacent to the entrance of the J-box. Some fabrics stick in the curved J and a ‘wet bottom’, formed by introducing water into a sump at the bottom of the curved section, is usually used to correct the problem. The water not only acts as a lubricant but also as an initial wash. To produce the wet bottom there is often a stand-pipe through which water flows at about 40 l min–1. The stand-pipe can be swivelled to control the amount of liquor in the J-box. Rope J-boxes hold 5–20 km of fabric for a dwell time of 1–2 h at a running speed of 100–150 m min–1. This converts into a productivity of about 130 kg h–1. Surface-sensitive fabrics, such as satins and sateens, and cotton/ polyester blends are prone to creasing under such conditions. Heat setting of polyester/cotton prior to rope preparation reduces the problem, but such blends are best processed in open-width form. The open-width short, or broken back, J-box (see Figure 2.6) provides a dwell time of 8–20 min and has a much smaller capacity than the rope J-box, in order
  • 109. 102 PREPARATION HomeContents to minimise the frequency and intensity of fold marks caused by the weight of fabric moving through the J-box. It is important with this equipment to ensure that the metal of the box does not become overheated. If this does occur, the selvedge areas of the fabric that have touched the sides of the box can show different dyeing properties from the rest of the material. The FMC continuous kier evolved from the wet-bottom J-box [43]. Here the water flow is replaced by bleach liquor which is circulated, by pump, from a heel tank, at a concentration about 30–50% of the saturator concentration, to the J- box. This used to be the most popular method of bleaching tubular knitgoods in the USA but seems to be being replaced by spiral-type winch machines. Steamers The first open-width steamers to be used in preparation were the atmospheric tight-strand roller steamers used for continuous dyeing. Kirner, in a classic paper [44], advocated such steamers for bleaching and scouring under shock conditions. As the fabric is tightly in contact with the rollers, an increased dwell can only be achieved by reducing line speed. At the lowest line speed, the dwell time is only 1–2 min; often too short to remove seed from woven cotton fabrics. In Europe, other designs of roller steamer were developed to try to increase the dwell time without reducing line speed. The roller-bed steamer, which consists of a flat bed of rollers driven much slower than the line, emerged as the most popular. Fabric is plaited down on to the slowly moving roller bed, which transports the fabric in a relaxed state to the exit point (Figure 2.7). It is possible to maintain a required production speed while varying the retention time in the steamer by adjusting the amount of fabric plaited on to the bed and consequently the amount of fabric in the steamer. This leads to a risk of trapping the fabric on the bed. Küsters, an innovative machinery maker, overcame this by plaiting the fabric on to a roller rather than the bed. As the roller rotates to deposit the fabric on the bed, the pile inverts [37,38]. Early machines provided dwell times of 3–5 min, but even under these conditions on the roller bed, some creases were formed on polyester/cotton fabrics. These creases are prevented by using a short tight-strand section, to allow swelling and dimensional changes to occur before plaiting down on to the bed. This combination is used in the second generation of roller-bed steamers, which are thus called Combi-steamers. Dwell times have also been extended to 10–15 min, which aids seed removal in a combined scour/bleach process. Good reviews of the development and performance of steamers have been given by Theusink of Brugman [45] and Tischbein of Babcock [46].
  • 110. 103 HomeContents PREPARATION MACHINERY Multi-layer conveyor steamer U box Roller-bed steamer Figure 2.7 Open-width steamers for fabric preparation: (a) roller-bed steamer; (b) conveyor steamer. (Source: [34].) The conveyor steamer was introduced to the USA in 1938 by Mathieson Alkali for chlorite bleaching. The impregnated fabric enters a steam-heated chamber and is plaited on to the first of three conveyor belts moving along the length of the machine (Figure 2.7). At the end of the first traverse the fabric is dropped onto a second conveyor and after another traverse onto a third conveyor. A temperature of 95°C and dwell times of 15–60 min are typical. Single-layer conveyor steamers, developed by Morrison Machinery Inc., are more popular in the USA for bleaching woven goods than roller-bed steamers. Brugman conveyors are used for bleaching linen and linen union fabrics, where the long dwell time makes them attractive, and a Fleissner conveyor is used for a multiple-strand pad–steam process for knitgoods. In the early 1960s, James Hunter developed a commercial pressure-scouring unit in which fabric entered and left the steamer through roller seals.
  • 111. 104 PREPARATION HomeContents Kleinewefers and Mather Platt produced machines employing lip seals which overcame the problems of roller seals. The lip seals used a pressure tube held against a metal plate, with the fabric passing between the tube and plate without loss of vessel pressure. In the Mather Platt machine, with its Vaporloc unit, the fabric entry and exit are side by side, and the fabric is plaited onto a roller bed. The Kleinewefers machine has entry and exit points at opposite sides of the pressure unit and is a tight-strand steamer in which expander bars are used to prevent crease formation. Normal dwell times vary in the range 1–3 min, with production speeds of about 100 m min–1. Normally the capacity of the unit is 60 m which gives a dwell time of 1 min when running at 60 m min–1. The machines, despite their productivity, are not very popular. Storage at long liquor ratio Knitgoods are often stored in the bleach liquor during continuous processing. These devices include under-liquor or immersion bleaching machines, as well as spiral winches or jets. In the spiral machines, the fabric rope is fed into the machine at one end, moves through the machine in a spiral path and is then unloaded at the other end of the machine. The immersion or PKS bleaching process developed by Bayer [47], provides continuous bleaching under long liquor conditions (between 25:1 and 30:1). Fabric is introduced, in open-width or rope form, into the reaction box under the liquor surface and transported through the liquor to give a dwell time of about 10 min at 90–95°C. Bleach liquor is constantly recirculated and provides a transport medium for fabric in the reaction chamber. The system has high energy and chemical costs compared with most continuous processes, and these, coupled with a notoriously unstable bleach bath formulation, have limited the acceptance of this technique, but machines made by German manufacturers such as Goller, Menzel and Thies have all run satisfactorily. In terms of spiral winches (the fabric is moved by winch reels) the machine made by Brückner, the Colorado, is probably most universally known, but the machines made by Jacumin Engineering Co. (JEMCO) called JEMCO II and III are probably best known in the USA. The Colorado is now seldom used for bleaching and is preferred for high-efficiency washing-off, so Brückner introduced the Tubolavar and Delphin as replacements. The most popular spiral jet, especially in the USA, was the Argathen. The machines are now made and sold by Küsters, who are not only continuing to support the US market but also introducing them into Europe.
  • 112. 105 HomeContents PREPARATION MACHINERY Chemical addition to saturators [48,49] Fabric passing continuously through a saturator not only takes liquor out of the system but also consumes chemicals. This is not a problem if dry fabric is being impregnated (wet-on-dry) as the feed liquor, usually located in a 5000 l tank in the dye kitchen, will be at exactly the same concentration as the saturator liquor and is controlled by a simple ballcock in the saturator. In this case the percentage chemical on weight of fabric (% o.w.f.) is given by Eqn 2.1: % o.w.f. = g l −1 × % pick-up/1000 (2.1) If the substrate is wet from the washer of a preceding stage, for example, then not only is liquor taken out of the system but water is brought into the saturator by the fabric. In theory, it is possible to measure the amount of water carried over (input) and the amount of liquor taken out (output) and so calculate the volume change in the saturator. The simple percentage pick-up in the relationship above is replaced, in this case, by percentage effective pick-up. This is the difference between the two pick-ups plus the fraction of the incoming water that has been exchanged for liquor. It is also possible to measure the concentration of chemical remaining on the substrate and to calculate the concentration of a feed liquor to replace the volume loss and compensate for the diluting effect of the incoming water [49]. In practice, the feed liquors would be chemicals at the concentrations supplied by the manufacturer, such as 46% by weight caustic soda liquor (100° Tw), and these would be metered to a mixing vessel and then added to the saturator, which has been brought back to volume by a ballcock fitted to the water supply pipe, at such a rate as to maintain chemical concentrations. Chemical dosing can be achieved by: (1) flow-controlled gravity feed using manually operated valves and rotameters or computer-controlled magnetic flow meters; (2) metering pumps (e.g. Bran and Lübbe, MPL) which have pistons of various sizes and vernier-controlled delivery adjustment; (3) measuring vessels (e.g. Texicon, ATC) which fill and empty with precalibrated volumes of feed liquors at a rate necessary to maintain saturator concentration; (4) electrode-based systems (e.g. Polymetron, Tytronics, ATC Bleach-o-matic) which measure saturator concentration directly and then open and close valves after comparison of the measured value with the desired value. All methods, excepting manual control, are interlocked to the main drive motor so that when the line is stopped, the chemical dosing ceases.
  • 113. 106 PREPARATION HomeContents 2.6 COTTON PREPARATION Preparation of woven fabrics normally consists of three stages: desizing, scouring and bleaching. In most other cases, preparation is a two-stage process: scouring and bleaching. For certain end uses, an extra stage (mercerising) can be included for yarns or fabrics. Woven fabric also requires singeing, prior to desizing. In this chapter, bleaching will be dealt with under a separate section. 2.6.1 Singeing [1] Many woven fabrics are singed to remove surface fibres. The fabric from an accumulator is passed through a brushing unit and then into a singeing machine. Various systems have been devised, for example those by Ostoff-Senge, Menzel and Parex Mather, the most usual having gas-fired burners through which the fabric passes at high speed in such a way that both sides of the fabric are singed. The fabric is passed through steam or liquor to quench any sparks. When a liquor is used it is often a desizing liquor. Singeing blends, for example polyester/ cotton, can create problems by melting the thermoplastic fibre. 2.6.2 Desizing [2] Sizing reduces the frictional properties of warp yarns by coating them with film- forming polymers. The process improves weaving productivity by increasing weft insertion speeds and decreasing yarn breakages. The size is usually applied from aqueous solution and film forms on drying. The following are typical sizing materials [5,50,51]: (1) Natural starches from potatoes, maize (corn), rice or tapioca. (2) Chemically modified starches (ethers or esters). (3) Organic polymers, e.g. polyacrylates, carboxymethylcellulose, methyl- cellulose, polyesters or poly(vinyl alcohol). (4) Solvent-soluble materials, e.g. copolymers of methyl methacrylate. Starch-based sizes are normally used for 100% cotton yarn as they are economical and produce satisfactory weaving performance. Other products are also used, either alone or in combination with starches, when the higher cost can be offset against improved weaving efficiency. For example, poly(vinyl alcohol) is widely used for sizing cotton/polyester blends. Size plasticisers, e.g. tallow or paraffin wax, may be added to the sizing mix or more usually applied after drying by kiss–roll coating.
  • 114. 107 HomeContents COTTON PREPARATION Most sizes are easily dissolved in water but on forming films they become much more difficult to redissolve. Singeing or heat setting can impair the water solubility even further. Desizing is often easier in a vertical organisation, since there is complete control of the choice of size and this often leads to size recovery processes. In most cases, however, the fabric will be imported and the finisher seldom knows the source of fabric, let alone how it has been sized. This leads to trial and error desizing – an unreasonable situation now resolved by oxidative desizing (see below). Size removal depends essentially on the following factors: (1) Viscosity of the size in solution. (2) Ease of dissolution of the size film on the fibre. (3) Amount of size applied. (4) Nature and amount of the plasticisers. (5) Fabric construction. (6) Method and nature of washing-off. (7) Temperature of washing-off. In the earliest processes the fabric was wetted and airborne organisms allowed to degrade the size (rot steeping). Acid treatments also degraded starch-based sizes and offered the further advantage of removing calcium and magnesium salts. Hydrochloric acid was preferable but all too often sulphuric acid was used on cost grounds. The concentration of acid ranged from 2% for short steeping times to 0.2% for overnight steeping. Care had to be taken to avoid any drying out as this would have damaged the cellulose itself [34]. These processes have been superseded by enzyme treatment, oxidative desizing or some form of washing-off procedure. Enzyme desizing [2,3] Enzyme desizing is widely practised and is highly effective for degrading starch or starch-based sizes. The enzymes used are the α-amylases and they are frequently applied in the quench box of the singer after singeing, but may be used on almost all types of batch and continuous equipment. The various preservatives (halogenated phenols) added to size mixtures to prevent mildew formation on damp fabric are toxic to enzymes, as are most anionic surfactants. As is usual in most chemical reactions, increasing temperature of treatment promotes increasing reaction rate and decreasing treatment time. The amylases may be obtained from specialist producers such as Novo Nordisk AB or Diamalt
  • 115. 108 PREPARATION HomeContents GmbH, but the auxiliary suppliers often have their own ranges. The enzyme may be a malt diastase, or a pancreatic or bacterial amylase. The optimum conditions for using the various types are shown in Table 2.3. The amount used depends on the activity of the product and there are at least two systems for measuring that activity. Usually the producer carries out such assays and builds the results into the application formulations. Compare the following recommendations for Aquazyme 120 L from Novo Nordisk AB (‘120’ refers to the strength of the enzyme): Table 2.3 Optimum conditions for enzyme desizing _____________________________________________________________________ Effects ofa ________________________ Optimum Optimum pH temp./°C NaCl Ca ions Time _____________________________________________________________________ Malt diastase 4.5–5.5 55–65 – + 12–24 h Pancreatic amylase 6.5–7.5 40–55 + + 12–24 h Bacterial amylase 6.5–7.5 65–75 + + 1–4 h Bacterial amylase 7.0–8.0 100–120 + + 1–2 min _____________________________________________________________________ a + indicates improved desizing, – indicates no effect. The enzymes cleave the α-1,4-glycosidic links between the glucose rings of starch. For amylose, the straight-chain fraction of starch, the fragments are glucose, maltose or oligosaccharides, all of which are soluble in boiling water. In the case of the branched amylopectin fraction, the oligosaccharide fragments may not be soluble in boiling water but are soluble in mildly alkaline solutions. The ratio of the two fractions varies from starch to starch. Rice and tapioca starches contain more amylopectin and are thus more difficult to remove than corn starch. Starch sizes can be detected by spotting with a solution of iodine in potassium iodide and this has been converted into a quantitative test [52], but the results should be interpreted with care as oxidative damage also gives a blue colour with the reagent [53]. J-box 20–50 g l–1 Combi-steamer 20–50 g l–1 Quench 10–25 g l–1 Winch 2–10 g l–1 Jig 5–15 g l–1
  • 116. 109 HomeContents Oxidative desizing Oxidants have been used for many years as desizing agents. Sodium hypochlorite desizing, prior to hydrogen peroxide kier bleaching, was particularly useful. Though less effective than enzymes for degrading starches fully, it gave some improvement in colour and allowed the peroxide bleach to complete size removal. The use of sodium bromite for desizing was developed in France [54]. It is claimed to provide a degree of bleaching with starch degradation, but has not been adopted commercially. It was highly sensitive to purity of the auxiliaries and for this reason has not been adopted. Oxidative desizing usually refers to the addition of peroxodisulphates (persulphates) or hydrogen peroxide to alkaline scouring liquors and so this form of desizing is discussed more fully in section 2.6.3. Hydrogen peroxide was developed initially for desizing fabrics sized with poly(vinyl alcohol) where desizing at about pH 9 is recommended. With mixed sizes a higher pH can be used [55]. Oxidative desizing is increasing in popularity: not only does it remove most sizes, but time is saved by removing process stages and, with hydrogen peroxide, it gives a degree of whitening. Hot caustic soda treatments Starch-based sizes can be removed by hot caustic soda treatments and it is possible to combine size removal with grey mercerising [56], but this precludes recycling the mercerisation liquor. Care must be taken when using sizes containing poly(vinyl alcohol) as caustic soda treatments can fix the size. This difficulty is overcome by using the oxidative desizing treatment. Caustic soda cannot be used to remove polyester sizes, e.g. Eastman Size WD, as hot alkaline treatment hydrolyses the polymer and leaves insoluble oligo-ester fragments on the cloth. Hot washing with detergents Simple aqueous washing removes some synthetic sizes. Sizes such as carboxymethylcellulose, poly(vinyl alcohol) and the polyacrylates swell when immersed in water. If this swollen size is agitated on a high-efficiency washer, complete size removal can be achieved. This is done in most countries in spite of the availability of size recovery techniques. In the USA, most of the large vertical companies are geared to size recovery COTTON PREPARATION
  • 117. 110 PREPARATION HomeContents where the swollen size is squeezed off at a high-pressure nip, prior to washing, and sent to a tank for shipping back to the weaving division. For these companies the large volume of the storage tanks for the recovered size is something of an embarrassment. A particularly attractive feature of recycling is the reduction in BOD from the desizing effluents. Solvent-soluble sizes Several companies are active in the development of solvent-soluble sizes, which are applied and removed in non-aqueous media. Though they have the advantage of eliminating aqueous pollution, economic and technical factors have limited progress. The technology has been reviewed in [50]. 2.6.3 Scouring Section 2.2 described the impurities present in cellulosic fibres. The purpose of scouring, or treatment with hot alkaline solutions, is to remove these impurities. The nature of these impurities and the process of scouring has been well reviewed [51]. In batch systems, scouring is often called boiling off if it is carried out below 100°C. For natural cellulosic fibres, the main effects of scouring are a 5–10% loss in weight and a dramatic improvement in wettability and absorbency. The loss in weight results from degradation of the proteins to amino acids, conversion of the alkaline earth pectates to soluble sodium salts, dissolution of the hemicelluloses and some degradation and dissolution of the cellulose. Lignins are degraded or dissolved. In the case of linen they are usually extracted as chlorinated lignins, as the scour follows a chlorination stage. The improved absorbency results from saponification of the fatty esters and melting of the fatty alcohols and hydrocarbon waxes at scouring temperature. These are emulsified by the fatty acid soaps formed during saponification. The removal of cotton wax need not be complete but has to be reduced to a level where it cannot form a continuous film over the fibre. In batch systems, wax removal increases with temperature. Figure 2.8 shows the relationship between absorbency and scouring conditions on the J-box [57]. The alkali most generally used is sodium hydroxide but other alkalis and the other additives to the scouring liquor were discussed in section 2.3.3. The use of a reducing agent under hot alkaline conditions not only minimises oxidative damage to the cellulose but also destroys coloured impurities. Traditionally, woven fabrics were batch scoured in kiers (kier boiled) and this
  • 118. 111 HomeContents COTTON PREPARATION process has been reviewed by Garrett [29]. Such machines are still used, for example in India and Pakistan, but have been almost universally replaced by J- boxes or Combi-steamers elsewhere. Kier boiling provided the thorough scouring which was essential before bleaching with hypochlorite but which is not so critical before bleaching with other agents. The processes most like kier boiling are currently carried out on loose stock. These conditions are the most severe as the fibre contains the highest level of impurities, and scouring above 100°C is absolutely essential for good absorbency. Knitted fabrics are often scoured on winch or jet. In this case, the yarn is usually clean, combed fibre and so scouring can be a rapid, mild process [31]. Yarn is scoured, or more usually boiled off, on hank or package machines. It must be noted that in any scouring process large amounts of impurities are dissolved or suspended in the scouring liquor. This is particularly important in batch processes as rapid cooling or pH change can cause deposition of the impurities on the substrate. Saturators can, in a similar way, build up large quantities of impurities and so filters on the circulation loop must be checked regularly. Antifoams are best avoided as they destabilise the liquor. Typical formulations and conditions for batchwise and continuous processes are shown in Table 2.4 [51]. There are many references to the ‘residuals’, contaminants on scoured textiles, but the figures quoted by Bell and Stalter provide indications of the expected levels (Table 2.5) [58]. Persulphates, and recently perphosphates, are recommended for use in caustic scouring processes to increase starch degradation. The perphosphates are more Figure 2.8 Absorbency and scouring conditions on a J- box [57]. 2 4 6 0 60 120 Caustic soda solution/% by weight Timeoftreatment/min Absorbent Non-absorbent
  • 119. 112 PREPARATION HomeContents Table 2.4 Batchwise and continuous methods of scouring (a) Batchwise methods ________________________________________________________________ Winch Package Kier or jet Jig or beam ________________________________________________________________ Caustic soda (100%)/g l–1 5–10 5–10 5–15 15–20 Wetting agent/g l–1 0.5–1 0.1–0.2 0.5–1 0.5–1 Detergent/g l–1 0.5–1 2–4 Liquor ratio 3–5:1 15–30:1 2–4:1 5–8:1 Temperature/°C 130 95–98 95–98 100–130 Time/h 4–6 1–2 2–4 3–0.5 ________________________________________________________________ (b) Continuous methods ________________________________________________________________ J-box Open-width conveyor atmospheric Pressure pad–roll steamer steamer ________________________________________________________________ Caustic soda (100%)/% o.w.g. 3–6 4–10 6–10 Wetting agent/% o.w.g. 0.1–0.5 0.1–0.5 0.1–0.5 Detergent/% o.w.g. 0.5–1 0.5–1 0.5–1 Temperature/°C 100 95–98 130–140 Time/min 30–120 10–30 1–2 ________________________________________________________________ Table 2.5 Change of average fabric characteristics on scouring and bleaching ____________________________________________________ Characteristic Grey Scoured Bleached ____________________________________________________ Whiteness/% 86.6 Whiteness (aged)/% 80.5 Whiteness retention/% 93.1 pH of aqueous extract 9.3 8.5 Fluidity/Pa s–1 2.2 2.9 5.3 Absorbency/s 3.0 Non-cellulosic content/% 9.9 0.99 0.39 Oils, fat and waxes/% 0.82 0.43 0.24 Ash/% 0.99 0.18 ____________________________________________________
  • 120. 113 HomeContents COTTON PREPARATION difficult to manufacture and are not generally available in commercial quantities. Under laboratory conditions it is not always easy to show differences in size removal with and without the addition of oxidants. In practice, however, their use gives improved uniformity of preparation and size removal. Oxidative desizing Oxidative desizing offers the possibility of reducing the number of processing stages required for fabric preparation, an important factor in minimising energy use. The oxidant can be added to caustic scour liquors to provide combined desizing and scouring. For this system little or no silicate or organic stabiliser is included. Alternatively some stabiliser may be added to give a degree of bleaching with desizing. This useful pretreatment extends the feasibility of combined scour/bleach processes, particularly when these would otherwise give inadequate bleaching and seed removal. In the general recommendations shown in Table 2.6, the lower levels of caustic soda provide desizing only while the higher levels provide oxidative scouring. The following should be noted: (1) Rapid desizing treatments require more critical control of alkali and oxidant concentrations. (2) Increased alkalinity for a given oxidant concentration tends to increase chemical damage. (3) Increased oxidant over the minimum required for desizing increases chemical damage. (4) Persulphates promote desizing rather than bleaching and require more critical control of concentration than does hydrogen peroxide. (5) Mixing persulphates and hydrogen peroxide is not recommended in pad– steam desizing. (6) To desize oxidatively by a batch process, the oxidant must be added when the alkaline fabric reaches top temperature. Solvent scouring If only fats and waxes were to be removed, this could possibly be achieved by treatment with organic solvent. Trichloroethylene was the first solvent used commercially, in a manner similar to that used for metal degreasing. Fabric was passed continuously through solvent vapour, which condensed on the fabric, dissolving out fats and waxes as it drained off. The system required special equipment which included solvent recovery. Even so, solvent losses contributed
  • 121. 114 PREPARATION HomeContents Table 2.6 Recommended conditions for oxidative desizing ___________________________________________________________________ Concn/% o.w.f. ___________________________________________________________________ Magnesium sulphate heptahydrate 0.005 0.005 0.005 Caustic soda (100%) 2–6 2–4 9–12 Stabiliser 0–1 0–1 0–1 DTPA (40% solution)a 0.2 0.2 0.2 Hydrogen peroxide (35%) 5–15 5–15 5–15 or sodium persulphate 0.2–0.5 0.2–0.5 0.2–0.5 Wetting agent 0.2–0.5 0.2–0.5 0.2–0.5 Temperature/°C 100 100 120–130 Time/min 15 15–60 1–2 ___________________________________________________________________ a DTPA – Diethylenetriamine pentaacetic acid significantly to process costs. Though fats and waxes were removed, additional wet processing was still required to remove other impurities. The Markal (ICI) system, whilst still requiring special plant, eliminated some of the disadvantages of solvent degreasing and combined the removal of fat and wax with starch degradation. In this process, fabric was immersed in trichloroethylene containing a detergent. The fabric was then passed into a steam box in which the trichloroethylene was removed by steam distillation. Condensation followed by water–solvent separation gave good, but not perfect, solvent recovery. Immersion in organic solvent gave penetration of the detergent, which remained in the fabric after steaming. Hot-water washing followed to remove fats and waxes. These had been made, in effect, self-emulsifiable. Introduction of an enzyme into the detergent solution gave starch degradation during steaming, allowing removal in the washing-off stage. The process provided fabric with excellent and consistent absorbency. The disadvantages were the high costs of equipment and solvent and the effluent problems, as all the residues remained in the wash-off liquor. To overcome the problem of solvent cost, processes were introduced which used paraffin hydrocarbons emulsified with aqueous enzymes. These emulsions were padded directly onto grey fabric and, after appropriate storage for size degradation, washed off. Good absorbency was obtained, but the effluent contained the hydrocarbons in addition to wax and size. These developments led to a better understanding about which detergents are most effective in removing fats and waxes during the pad application of desizing
  • 122. 115 HomeContents COTTON PREPARATION liquors. Whilst not strictly solvent emulsion processes, the use of these specialised auxiliary products has become common practice in many desizing and bleaching processes. 2.6.4 Mercerising [51] Treatment with strongly alkaline solutions to improve fibre lustre, tensile strength and dye uptake is called mercerisation. The name came from Mercer’s original invention based on treatment of cotton fabrics with strong caustic soda solution, without tension, to improve colour yield. It was Lowe who later found that the application of tension improved lustre. Mercerisation can be carried out either on yarn or on woven or knitted fabric. Yarn or fabric is normally impregnated with cold caustic soda 25–26% by weight solution (54–58° Tw) containing a good wetting agent. At this concentration considerable swelling of the cellulosic fibre occurs which results in shrinkage if the fabric or yarn is not held under tension. Mercerisation may be carried out on the grey, partially or fully prepared substrate. When mercerising loomstate fabrics, penetration by concentrated caustic soda at low temperature is difficult and tends to give surface effects. Grey goods also cause fouling of the liquor by size, making caustic recovery and recycling difficult. However, it is better to impregnate certain fabrics with caustic soda while they have their maximum strength, and hence there is still a considerable call for grey mercerisation. Rösch, in his excellent series of articles on the practice of cotton fabric preparation, argues that the best position is after scouring and bleaching [59]. The weft threads of a woven fabric are kept directly under tension on a clip stenter (chain merceriser) or the fabric dimensions are controlled by a series of rollers (chainless merceriser). In adddition to these measures, the fabric is usually woven slightly wider to allow for some width loss. An important consideration in mercerisation is the rate of desorption of the caustic soda. Yarn is mercerised in hanks between two movable rollers which are used to apply tension. Knitted fabrics may be mercerised in slit or tubular form [51]. As well as the effects mentioned, mercerisation also improves fibre and fabric smoothness, coverage of dead cotton and dimensional stability. All properties are influenced by alkali concentration, temperature and dwell time in alkali prior to washing-off [56]. Usually, mercerisation is carried out using cold caustic liquors but some interest has been shown in the so-called hot mercerising process [60,61]. The basic principles are:
  • 123. 116 PREPARATION HomeContents (1) saturation with mercerising-strength caustic soda solution near to its boiling point; (2) controlled hot stretching; (3) controlled cooling; (4) traditional tension-controlled washing followed by final washing. The advantages indicated for the process are: (1) shortening of the processing sequence providing cost savings; (2) increased efficiency and uniformity; (3) use of chain or chainless equipment with less problems related to fabric width; (4) improved lustre, tensile strength and dimensional stability since greater fabric stretching is possible; (5) increased dye uptake, though this is dependent on the extent of stretch imparted; greater than normal stretch reduces the dyeability with greater internal orientation of molecular structure; (6) good response from fabrics containing lower-grade cotton; (7) flash scouring effect obtained; (8) good desizing; (9) better fabric penetration by a hot caustic liquor than with normal mercerisation. The main chemical and physical changes do not take place at the initial elevated temperature, but when the cooled fabrics go through the traditional section of the mercerising plant. The possibility of a combined scouring/mercerising process is of considerable potential interest, particularly as the degree of scouring, as a preliminary to peroxide bleaching, is said to equal that of a conventional caustic scour. However, since its introduction in 1977, this development has not achieved significant commercial success. 2.6.5 Liquid ammonia treatment [51,62] Treatment of cotton with anhydrous liquid ammonia has also been developed to enhance the properties of cotton. In the original Prograde process, used on yarns, the ammonia is removed subsequently in a hot water bath. This treatment brings about an increase in tensile strength, lustre and dye substantivity. The improvements in lustre and dyeability are less than when mercerising with caustic soda, although the ammonia-treated cotton has greater resistance to thermal degradation.
  • 124. 117 HomeContents In fabric processing the ammonia can be removed by dry heat. This technique gives properties quite different from the Prograde system and should not be considered to be a mercerisation process. Dye uptake may vary from a slight increase to a decrease, depending on the dye. However, the dry-heat removal does give improved crease recovery, and durable easy-care finishes can be obtained with lower resin additions. Hence the benefits are associated primarily with improved properties in resin-finished cellulosic fabrics; the ammonia treatment may follow dyeing. 2.7 BLEACHING All natural fibres are coloured and the colouring matter confers a yellowish brown colour to the fibres. In the case of cotton, this is believed to be a condensation product of caffeic and quinic acids which is related to chlorogenic acid [63]. The purpose of bleaching is to destroy this coloured material and to confer a pure white appearance to the fibres. Bleaching should also decolorise or remove any residual impurities left by scouring. Bleaching is the linchpin of preparation and today really means bleaching with hydrogen peroxide (peroxide), since sodium hypochlorite (hypo) and sodium chlorite have both lost ground to peroxide. Hypo is increasingly unacceptable environmentally; chlorite is difficult to use (special materials of construction, extraction system for chlorine dioxide, etc.) and was caught by a change in fashion – supersoft finishing – which made a non-scouring bleaching process redundant. Chlorite bleaching plant using stoneware was still sold until quite recently [14], and both the plant and the chlorite are advocated as environmentally acceptable [64]. 2.7.1 Bleaching with sodium hypochlorite [5,51] Effect of pH Figure 2.9 shows how the species in solution change with varying pH of a sodium hypochlorite solution. Figure 2.10 illustrates the classical fluidity results of Clibbens and Ridge which show maximum chemical damage to the cellulose occurs at pH 7 (see section 1.5.5). The bleaching effect is highly dependent on pH and can be represented by Scheme 2.2. As the curves show, reaction 1 predominates above pH 10, whilst below pH 3 reaction 3 predominates. Between pH 4 and 8 the hypochlorous acid predominates. It would appear, therefore, that it is this species that is damaging to cellulose. This is a factor which must be considered when washing off after hypo bleaching. The liberation BLEACHING
  • 125. 118 PREPARATION HomeContents Figure 2.9 Effect of pH on the composition of sodium hypochlorite liquors. (Source: Agster, Textilveredlung, 1 (1966) 276.) 2.5 4 6 8 10 8 12 16 20 24 pH Fluidityof0.5%soln Figure 2.10 Effect of pH on degradation of cellulose by sodium hypochlorite. (Source: [3].) of chlorine can be helpful (section 2.9.2) but chlorination of organic materials leads to AOX and it is for this reason that hypo will go out of use [13]. Effect of temperature and time Increased temperature gives a higher rate of bleaching. This relationship was studied by Derry [65], who showed not only that a 6 h bleach at 20°C could be re- duced to about 7 min at 60°C, but also that an increase in temperature brings a real danger of severe 0 4 8 12 0 50 100 pH Concn/% Cl2 HClO ClO–
  • 126. 119 HomeContents BLEACHING NaOCl Na+ + OCl– 1 HOCl H+ + OCl– 2 HOCl + H+ + Cl– Cl2 + H2O 3 Scheme 2.2 chemical damage. Bleaching is usually carried out for several hours at ambient temperature (less than 20°C). Increasing temperature can lead to loss of active chlorine from both the storage and bleaching vessels. This situation leads to variable bleaching in many countries during the summer. Other methods are available for accelerating the hypo bleach, including irradiation with u.v. [51] and the addition of cobalt [11] or water-soluble polyhydroxy compounds. Effect on cellulose Epstein and Lewin showed that hypo oxidation of cotton in the pH range 5–10 forms more carboxy groups and less aldehyde and ketone groups as the pH increases [66]. They proposed oxidation of the C2–C3 linkage within the glucose units which sensitises the 1,4-glycosidic linkages between units and makes their cleavage easy (β-hydroxycarbonyl elimination or peeling reaction) at any point in the chain, leading to rapid depolymerisation. This fits well with the Clibbens and Ridge results. This mechanism also suggests that the fragments (furfuraldehyde derivatives) make hypo-bleached cotton more prone to yellowing with time (see section 1.5.5). Hypochlorite bleaching processes Hypo liquors should always be titrated before use. Formulations are usually expressed as active or available chlorine (AC), as this gives a fixed datum. It is necessary to convert this fixed value to material as supplied before use. This is done using Eqn 2.2: Volume of hypo as supplied = 1000 × desired AC in bleach bath AC of hypo as supplied (2.2) This volume would be added to the bleaching vessel or a side tank containing water.
  • 127. 120 PREPARATION HomeContents Alkali, normally soda ash, is added, predissolved in a bucket and the pH checked. It is usual to check available chlorine and pH during the course of bleaching. The traditional process is cistern bleaching, which may be regarded as an open kier as far as liquor circulation is concerned but the process can be carried out on various batch machines [5]. Cisterns are usually cylindrical stoneware vessels but tile-lined pits were often used in their place. Typical formulations are shown in Table 2.7 [51]. A J-box in the form of an open-sided plastic piler has been used, for example in Eastern Europe and India, at higher concentrations (7–10 g l–1 available chlorine) as a rapid stage prior to peroxide bleaching [67]. The advantages of bleaching with hypochlorite are: (1) low chemical cost; (2) lower (not zero) risk of catalytic damage; (3) low energy input (heating cost). The disadvantages of bleaching with hypochlorite are: (1) the formation of high levels of AOX and chloroform; (2) there is no rapid bleaching process possible; (3) the danger of yellowing of the bleached fibre on storage; (4) the danger of chemical damage to the cellulose (temperature and pH); (5) the fibre must be prescoured before bleaching; (6) degrades most dyes and fluorescent brightening agents (FBAs). 2.7.2 Bleaching with sodium chlorite Effect of pH The bleaching activity of sodium chlorite depends strongly on pH. Figure 2.11 Table 2.7 Recommended conditions for hypochlorite bleaching __________________________________________________________________ Package J-box Cistern Jig Winch machine conveyor __________________________________________________________________ Available chlorine/g l–1 2–4 2–4 1–2 1–2 2–5 Soda ash/g l–1 2–4 2–4 1–2 1–2 2–4 Time/h 3–4 1–2 1–2 1–2 1–2 __________________________________________________________________
  • 128. 121 HomeContents BLEACHING Figure 2.11 Effect of pH on the composition of sodium chlorite liquors. (Source: [98].) 0 2 4 6 8 10 pH Concn HClO3 ClO2 ClO2 – HClO2 50 100 5ClO2 – + 2H+ 4ClO2 + Cl– + 2OH– 3ClO2 – 2ClO3 – + Cl– ClO2 – Cl– + 2[O] Scheme 2.3 shows the species in solution as the pH of a sodium chlorite solution changes. The reactions occurring in solution are highly complex and have been reviewed [2,51]. According to several authors, the chlorous acid (HClO2) formed in solution by hydrolysis of the sodium salt undergoes the series of reactions in Scheme 2.3. In order to achieve the necessary concentration of chlorous acid, acid or latent acid generators (activators) must be added to the bleaching solution [68,69]. Activators include sodium chloroacetate, chloroacetamide, triethanolamine and ammonium persulphate [70]. It is also possible to activate sodium chlorite by irradiation with u.v. Bleaching occurs most rapidly below pH 1–2, rapidly at pH 2–3 and the rate decreases as the pH increases to 8–9. Between pH 2 and 9 this rate is
  • 129. 122 PREPARATION HomeContents proportional to the concentration of chlorous acid in solution [51]. Below pH 2 the species providing the bleaching action is chlorine dioxide, a toxic, explosive, corrosive gas. In practice, a balance is usually struck between the rate of bleaching and the evolution of chlorine dioxide by controlling the bleach liquor pH. This is conveniently done on batch machinery by adjusting the pH to 3.5– 4.5 with acetic or formic acid and then buffering at this pH by addition of sodium dihydrogen phosphate. The phosphate addition not only buffers the system but also improves whiteness. When bleaching at low liquor ratio, for example on package machines or continuous ranges, the initial pH may be set higher at 4.5–6 since impurities in the substrate, especially if it is unscoured, liberate acid during bleaching, which tends to push the pH into the optimum range. The chlorine formed (Scheme 2.3) does form some AOX but only about a tenth of that from hypo [13]. There is some discussion, however, on the source of this AOX. Effect of temperature and time Van’t Hoff showed that, in most chemical reactions, the reaction rate doubles with each 10 degC rise in temperature. Chlorite bleaching is no exception. There are no reported detrimental effects of bleaching with sodium chlorite around 100°C, or of bleaching for more than several hours. Sodium chlorite will bleach cellulose at ambient temperature but does need an overnight dwell. When bleaching is carried out at 100°C this dwell time is considerably reduced, for example to 1–4 h. Unlike hydrogen peroxide, no rapid (10–20 min) bleaching processes are possible. Effect on cellulose It is generally accepted that, under optimum bleaching conditions, sodium chlorite does not degrade cellulose. When the chlorite is in large excess or at pH above 7, however, the cotton is degraded [51]. Little is known about the functional groups formed by chlorite oxidation of cellulose. Chlorous acid at pH 3 quantitatively oxidises aldehydes or other carbonyl groups but the results are concentration-dependent. For example, if a C1 aldehyde is oxidised, a gluconic acid ester is formed. With 0.2 mol l–1 chlorous acid there was no oxidation but a 1 mol l–1 solution gave extensive degradation after long dwell times [51]. Chlorine dioxide seems to oxidise cellulose in a similar way to chlorous acid.
  • 130. 123 HomeContents BLEACHING The presence of carbonyl groups in cellulose pulp bleached with chlorine dioxide seems to be responsible for the yellowing and solubility in hot alkali of the pulp [51]. Chlorite bleaching processes It is usual, though not essential, to pretreat before chlorite bleaching. The procedure depends on the machinery used, the form of the textile and the end use of the product. Woven fabrics would normally be desized and scoured whereas yarns and knitted fabrics would simply be washed off prior to bleaching to remove spinning or knitting lubricant. Loose stock would normally be required for an absorbent end use and so would need a full caustic scour before bleaching. Because of the danger from chlorine dioxide evolution, hooded machines connected to an extraction system are usually required and even then the area should be well ventilated. Typical formulations are shown in Table 2.8 [51]. Cistern bleaching is often carried out on the equipment made by Friedrichfeld GmbH. This is widely covered elsewhere and is often described as a pack system [14]. Though usually promoted for knitted fabrics, it has been used for linen and linen union woven fabrics. On cistern, package machine, beam-dyeing machine, jet and possibly winch (in older dyehouses, chemicals would be added directly to the stuffer box of the winch machine), the bleach chemicals are made up in a sidetank adjacent to the machine or in the dye kitchen. The sidetank is about half-filled with cold water and then the sodium nitrate, buffer salts or activators, surfactants and sodium chlorite are added with stirring. Any solids should be predissolved before use. Finally, dilute acid is added and the pH checked and adjusted, if necessary. The Table 2.8 Recommended conditions for chlorite bleaching ___________________________________________________________________________________ Winch Package J-box Cold Cistern Jig or jet machine conveyor bleach ___________________________________________________________________________________ Sodium chlorite (80%)/g l–1 2–4 2–6 1–3 2–4 20 20–25 Sodium dihydrogen phosphate/g l–1 0.5–2 0.5–2 1 0.5–1 Sodium nitrate/g l–1 1–2 1–3 1–2 pH 3.8–4.2 3.8–4.2 3.8–4.2 3.8–4.2 6–6.5 6–6.5 Temperature/°C 80–85 80–85 80 80 80–85 20 Time/h 3–4 1–2 1–2 1–2 1–4 16–18 ___________________________________________________________________________________
  • 131. 124 PREPARATION HomeContents transfer pump is started and the contents of the tank transferred to the machine. The pump is then stopped and, after circulating the liquor in the machine for 5– 10 min, the pH is again checked and preferably the chlorite content titrated. The temperature is raised to 85–95°C at 1–2 degC min–1 and held at that temperature for the appropriate time. For jig bleaching the chemicals would be added directly to the jig in the order given. For J-box, conveyor or other pad–steam systems as well as for cold pad–batch bleaching, a similar system of tanks would be used for making up the pad liquor for the saturator. The actual concentration of this liquor will depend on whether the application is wet-on-dry or wet-on-wet. Aftertreatment, e.g. with 2–5 g l–1 detergent plus 2–5 g l–1 soda ash at 85– 95°C, on the bleaching machine, rope washer (Tensitrol or Colorado) or open soaper will often enhance whiteness and will usually improve absorbency of the substrate. An antichlor is only carried out for goods to be subsequently dyed. This can be accomplished by replacing the soda ash in the aftertreatment with sodium perborate or percarbonate but is more usually accomplished with sodium thiosulphate or oxalic acid. The reductive antichlors (hydros, sulphite or bisulphite) are seldom used as they replace one problem with another. For white goods, aftertreatment is often used to apply FBA and softener at pH 6–7. The advantages of bleaching with sodium chlorite are: (1) no need for severe precleaning treatments; (2) smaller weight losses due to non-scouring action of acid bleach; (3) soft fabric handle and good sewability due to non-removal of fats; (4) low chemical damage; (5) least sensitive to metallic contamination; (6) bleaches synthetic fibres; (7) acid process makes washing-off easier. The disadvantages of bleaching with sodium chlorite are: (1) a possibility of liberating the toxic gas, chlorine dioxide; (2) the equipment is expensive because of the need for exotic construction materials; (3) no rapid bleach process available; (4) poor absorbency due to residual fats and waxes; (5) expensive; (6) incompatible with most dyes and FBAs; (7) multichemical baths which need control.
  • 132. 125 HomeContents 2.7.3 Bleaching with hydrogen peroxide Hydrogen peroxide has achieved its dominant position as a bleaching agent because of three factors: (1) It is environmentally innocuous (potentially it can decompose into oxygen and water). (2) It is versatile (it can be used hot or cold, in rapid or long-dwell processes, batchwise or continuously). (3) A variety of activation routes is available. Hydrogen peroxide as supplied by the manufacturers is extremely stable. Typically, only a 3% loss of activity would result from storage at 40°C for 1 year [28]. On account of this innate stability, peroxide has only a slight bleaching effect on cellulose. For bleaching to occur this stability must be overcome by activation. Activators include alkalis, sulphuric acid, u.v. irradiation, hypochlorite, transition metals and O- and N-acyl compounds [71]. The usual activator for textile bleaching is alkali, usually sodium hydroxide, and the peroxide molecule undergoes heterolytic fission to the perhydroxide ion [72], as in Scheme 2.4. This nucleophile can react with organic compounds by a displacement reaction or by addition to a double bond. BLEACHING H2O2 + OH– OOH– + H2O Scheme 2.4 Reagents such as transition metals and u.v. irradiation cause homolytic fission of the peroxide into two hydroxyl radicals (•OH) and the problems created by these radicals have been reviewed [73]. It is probable that the pigments responsible for the natural colour of cotton contain a chromophoric system of conjugated double bonds. These will be attacked by a free-radical system and it has been proposed by Cates and Taher [74] that peroxide bleaching takes place by the mechanism indicated in Scheme 2.5. Such free radicals might be produced by reaction of hydrogen peroxide with an electron donor, possibly derived from a metal cation or from a perhydroxide anion. The reaction mechanism proposed is shown in Scheme 2.6. The free radicals are believed to initiate decomposition through a chain mechanism. The decomposition within a given time interval is limited by the concentration of coloured impurities or by inhibiting metal cations. The latter appear to protect the cellulose from chemical damage by minimising the formation of molecular oxygen in the alkaline medium.
  • 133. 126 PREPARATION HomeContents RR + Scheme 2.5 (a) H2O2 H+ + OOH– (b) H2O2 + M2+ M3+ + OH– + OH (c) H2O2 + OOH– OOH + OH– + OH (d) H2O2 + OH OOH + H2O (e) OOH + M3+ M2+ + H+ + O2 or Scheme 2.6 Since these mechanisms are not fully understood, the bleacher still relies on control systems developed empirically. This control is often termed stabilisation, but this should not be confused with the use of chemicals to give the commercial product stability during storage. These compounds are effective at about pH 4.5–5, the pH at which commercial hydrogen peroxide solutions are most stable. Correct process stabilisation conditions control the rate of bleaching and ensure a residual peroxide content prior to washing-off, while making the most economic use of the chemicals. In this way the required standards of whiteness with minimum chemical damage are obtained. In effect four parameters require to be balanced: time, temperature, alkalinity and stabiliser content. Each of these factors will now be considered. Effect of pH Hydrogen peroxide does not have the multiplicity of species in solution as pH changes and so it is not possible to show graphs similar to those for the ‘chlorine’ bleaches. Alkalinity does have an effect and this was elegantly demonstrated by Naujoks for winch bleaching (Figure 2.12) [75] and by Rowe for pad–steam bleaching (Figure 2.13) [43]. Stabilisers High pH and temperature lead to the decomposition of peroxide bleaching liquor and degradation of the cellulose, which are catalysed by transition metal ions [73]. The role of the stabiliser is simply to control or regulate these effects by a multiplicity of functions. For example, they act as buffers, sequestrants and dispersants as well as, in special cases, enhancing performance of the surfactants used in the bleach bath. The sequestering action inactivates metallic impurities which cause catalytic decomposition of hydrogen peroxide or precipitation of
  • 134. 127 HomeContents BLEACHING Figure 2.12 Effect of pH on whiteness for winch bleaching with hydrogen peroxide. (Source: [75].) Figure 2.13 Effect of alkali concentration on J-box bleaching. (Source: [43].) 0 2 4 6 73 75 77 79 NaOH/g l–1 Reflectance/% 0 4 8 87 88 89 NaOH/g l–1 Reflectance/%
  • 135. 128 PREPARATION HomeContents hydroxides or carbonates. These impurities, the most common being calcium and iron, are brought into the bleaching system by the fabric, water supply or the other chemicals used. The stabilisers first used in bleaching cellulosic fibres with peroxide were the sodium polysilicates [71,72,76], often called water glass, which are highly effective at low cost. Sodium metasilicate (Na2SiO3) may be used in some applications as it can be washed out more readily than the polysilicates. The use of sodium orthosilicate (Na4SiO4) has been described [77] and even potassium silicates have been proposed [78]. The ratio of Na2O to SiO2 (Table 2.9) is often used to distinguish between the sodium silicates. In using either metasilicate or orthosilicate, more Na2O is added than that provided by the polysilicates and so the Na2O provided by the alkali would need to be reduced to maintain the ratio. The stabilising action of the sodium silicates and other stabilisers is improved by addition of magnesium ions. These ions may be present in the cotton being processed, or as hardness in the process water. In the case of soft water, the addition of such ions is recommended. This is usually done by adding magnesium sulphate crystals (MgSO4.7H2O) to give a water hardness equivalent to 3–4° English hardness [54]. Table 2.9 Relative Na2O and SiO2 content of bleach chemicals _____________________________________________________ Product Na2O/% SiO2/% _____________________________________________________ Sodium orthosilicate(Na4SiO4) 67.4 32.6 Sodium metasilicate (Na2SiO3.5H2O) 29.0 28.2 Sodium silicate (sp. gr. 1.4) 8.8 29.0 Caustic soda 77.5 Soda ash 58.0 _____________________________________________________ Over the years there has been considerable discussion on the role of silicates in peroxide bleaching, particularly on their effectiveness to control transition-metal contamination. The following mechanisms have been proposed: (1) The silicate may form silica gel with high specific surface area or colloidal magnesium silicate by reaction with magnesium salts; either of these products can absorb metallic impurities. (2) Silicate itself can complex with hydrogen peroxide to form unstable peroxides.
  • 136. 129 HomeContents (3) Silicate may bind free radicals formed in the presence of some metallic catalysts, and whether the addition of calcium or magnesium salts supports this action depends on the nature of the metallic catalyst. (4) Silicate not only may stabilise the peroxide but may also increase the efficiency of bleaching through the formation of peroxosilicate. Most of the research work on the mechanism of stabilisation has been done under long liquor conditions. The proposed mechanism may not, therefore, be applicable to short liquor ratio systems. The simple explanation of magnesium silicate formation provides only a partial understanding of stabilisation. Certain auxiliaries promote its formation but prevent crystal growth and therefore precipitation [76]. When using silicates in peroxide bleaching it is important to establish the correct balance between the silica (SiO2) content derived from the silicate and the sodium oxide (Na2O) content derived from the alkali and the silicate. The proportions of Na2O and SiO2 shown in Table 2.9 may be used to calculate the total amounts in a bleaching liquor. In a typical long liquor bleaching formulation the calculation of the Na2O:SiO2 ratio is as shown in Table 2.10. Substrates bleached with a silicate stabiliser must be washed off in soft, neutral, boiling water in order to prevent problems caused by insoluble silicates or silicic acid. Although sodium silicate is both effective and economical, the problems associated with silicate deposits on fabric and machinery have made alternative non-silicate or organic stabilisers attractive. These products are blends of organic materials with or without magnesium salts, and are mainly of five chemical types: (1) Organic sequestering agents. (2) Protein degradation products. (3) Certain surfactants. (4) Polymeric materials. (5) Mixtures of any of the above. BLEACHING Table 2.10 Determination of Na2O:SiO2 ratio for long liquor bleaching ______________________________________________________________________________ Sodium silicate(sp. gr. 1.4) 7.0 g l–1 provides 2.03 g l–1 SiO2 0.62 g l–1 Na2O Caustic soda 0.5 g l–1 provides 0.39 g l–1 Na2O Soda ash 1.8 g l–1 provides 1.04 g l–1 Na2O Total SiO2 and Na2O is 2.03 g l–1 SiO2 2.05 g l–1 Na2O The above system therefore provides a 1:1 ratio of Na2O to SiO2. ______________________________________________________________________________
  • 137. 130 PREPARATION HomeContents Stabilisers may be formulated for stabilisation alone or combined with other products to confer detergency, softness, crease-free running, etc. They are particularly useful on package machines, to minimise filtration, and jigs, where washing-off is difficult. For processing knitgoods on winch or jet the product may also include alkali, allowing addition of a single product to the bleach bath. In general, washing-off is simplified by using organic stabilisers, since citric or acetic acid can be used to neutralise residual alkali with no danger of precipitating silicates. Originally the whiteness attained from bleaching with organic stabilisers was only considered acceptable when batch processing. Organic stabilisers were considered less suitable in pad–steam bleaching and it was believed that some silicate must be included. In recent years new products have been developed. These give similar results to silicate and are now used in continuous pad–steam processes to produce full whites without silicate addition to the saturator [79]. The degree of preparation prior to peroxide bleaching influences alkali and stabiliser concentrations used in the bleach bath. When pretreatment is limited to the oxidative removal of size the presence in the cotton of natural impurities helps the stabilisation and so the bleach liquor may need less stabiliser or more activator. Acid souring, to neutralise scouring or remove metallic contamination, can also remove natural calcium and magnesium ions from cotton and so, in this case, an increase in the magnesium sulphate addition may be necessary. Alkaline scouring may cause carry-over of alkali into the bleach bath and so the alkali in the bleach may need to be reduced or omitted altogether. Sequestering agents are often successfully used to control the effects of small quantities of transition-metal ions during bleaching. They are less effective when the contamination is present as highly localised concentrations such as rust particles. Polyphosphates and EDTA (section 2.3.4) are reported as eliminating or reducing the catalytic degradation resulting from metal ions in baths stabilised with silicate in the presence of magnesium ions [80]. Experience indicates that is not always the case, although it is acknowledged that silicates do play a major role in controlling catalytic metals during peroxide bleaching. In general, it is difficult to control catalytic damage from within the bleach bath and when it is suspected that it may occur, for example on Brazilian cotton contaminated with traces of manganese, an acid sour at long liquor ratio prior to bleaching is recommended. The use of milder bleaching conditions (lower temperatures, time, chemical concentrations) will always moderate the effect of metallic contamination.
  • 138. 131 HomeContents BLEACHING Effect of temperature and time As with sodium chlorite, bleaching rate increases with temperature. There are no reported detrimental effects of bleaching with hydrogen peroxide around 100°C or of bleaching for more than several hours provided all other things are equal, i.e. the system is well balanced (concentrations of oxidant, stabiliser and alkali are in the correct ratio) and free from heavy metal ions. Hydrogen peroxide will bleach cellulose at ambient temperature but needs an overnight dwell. When bleaching is carried out at 100°C this dwell time is considerably reduced (e.g. to 3–20 min): this ability to bleach in such short times at high temperature is unique to hydrogen peroxide and makes rapid bleaching processes possible. Effect on cellulose The main functional groups formed by cellulose oxidised by hydrogen peroxide are ketone groups, i.e. there are very few aldehyde or carboxy groups. These ketone groups do not promote yellowing or β-hydroxycarbonyl elimination (the peeling reaction) [51]. Unlike hypochlorite it does not rapidly depolymerise the cellulose but slowly oxidises terminal anhydroglucose units in almost a ‘wet’ combustion. There are no large organic fragments formed which can cause yellowing, so the stability of the white is good. Peroxide bleaching processes As with chlorite bleaching, pretreatment prior to bleaching is usual but not essential. The nature of the pretreatment depends on the machinery used, the form of the textile and the end use of the substrate. Woven fabrics would normally be desized and scoured whereas yarn and knitted fabrics would simply be washed off prior to bleaching to remove spinning or knitting lubricant. Loose stock would normally be required for an absorbent end use and so would need a full caustic scour before bleaching. Because of the diversity of application conditions it is convenient to divide the processes into batchwise and continuous. Batchwise processes Typical formulations are given in Table 2.11 [51]. The bleach chemicals are made up in a sidetank. This is about half-filled with cold water and then the
  • 139. 132 PREPARATION HomeContents Table 2.11 Recommended conditions for batch peroxide bleaching _____________________________________________________________________ Kier/ Jig/ Winch Package % o.w.f. % o.w.f. or jet/g l–1 machine/g l–1 _____________________________________________________________________ Magnesium sulphate heptahydrate 0.1 0.1 Wetting agent 0.5–2 0.5–2 Sodium silicate (79°Tw) 2–3 3–5 7 2–7 Organic stabiliser 1–1.5 1–2 0.5–2 Caustic soda (100%) 0.6–1.4 0.25–0.8 5–15 5–15 Hydrogen peroxide (35%) 3–5 2–5 5–15 5–15 Liquor ratio 4:1 3:1 15–20:1 8–10:1 Temperature/°C 95 95 95 90 Time/h 1–2 1–2 1–2 1–2 _____________________________________________________________________ magnesium sulphate crystals (magnesium chloride should not be used because of the corrosive effect of chloride ions on stainless steel), organic materials (surfactants, stabilisers), sodium silicate and caustic soda are added with stirring. It must be remembered that the specific gravities of both sodium silicate (79° Tw) and caustic soda (70 or 100° Tw) are much greater that 1 and these liquors sink to the bottom of the tank. Stirring is, therefore, an absolute necessity. Any solids should be predissolved. Finally, hydrogen peroxide is added and if required the concentrations of alkali and peroxide are determined by titration. This check of concentrations is essential for package machines that use air-cushion pressurisation. The transfer pump is started and the contents of the tank transferred to the machine, before the temperature is raised to 85–95°C at 1–2 degC min–1 and held at that temperature for 1–2 h. For jig bleaching the chemicals would be added directly to the jig in the order given. For kier bleaching, the transfer pump supplies the piler and when the kier is about half full, the steam to the kier heater is opened to give a temperature of 40–50°C and loading is completed. The temperature is raised to 65–70°C over 30 min and the kier is rested for 30 min at this temperature to expel air. After raising to 80°C and resting again, the lid is closed and the temperature raised and held for 1–2 h at 95°C. On package machines with stock tanks, the spent bleach liquor can be pumped up and regenerated by bringing back almost to volume and again titrating (titre T1). If the original liquor made up had titre T0, then chemicals are added in the ratio defined by Eqn 2.3:
  • 140. 133 HomeContents Chemical addition (kg or l) = original addition × (T0 −T1 ) T0 (2.3) On a jig, water is normally used in the first few ends to bed in the fabric but can also be used for precleaning at the boil with detergent, enzyme, etc. At the bleaching step, all of the bleach chemicals, except peroxide, are added and the temperature raised to boil. To prevent variation from one end of the batch to the other (ending), the peroxide is drip fed over two to four ends. To moderate activation of the peroxide over the first end the peroxide should be added below 50°C. Washing-off is notoriously difficult on jigs, and overflow rinsing or soda ash scaling should be considered as solutions to problems at wash-off. In the latter, 2% soda ash on weight of good is added to the first wash liquor, the temperature raised above 80°C and the fabric run for a further two ends. This process not only gives an alkaline scour but also activates any residual peroxide which enhances the final whiteness of the fabric. Continuous processes [35] Continuous hydrogen peroxide bleaching of woven fabric in rope form was introduced in the late 1930s by the American peroxide producers Becco and DuPont. Many machine developments have been made since that time [35], and now most woven and, increasingly, knitted fabrics are bleached with hydrogen peroxide by continuous methods either in rope or open-width form. The main justification for installing a continuous range is productivity. Most knitted fabrics are scoured and bleached in a single stage on spiral rope processing machines such as the Küsters KRP, Jemco or Ashby Industries machines, but most woven fabrics are pad–steam processed continuously in three stages: desizing, scouring and bleaching. As the washing stages of such a route consume 75% of the energy, much attention has been given to combining process stages to reduce energy demand and minimise capital investment in plant. The scope for combining stages depends on grey fabric quality and the standard of preparation required, particularly seed removal. With a good balance between stabilisation, alkalinity, peroxide concentration, time and temperature [81], it is often possible to reduce the stages from three to two, or even one. The most usual limitation is inadequate seed removal in the absence of an alkaline pretreatment. Oxidative desizing with hydrogen peroxide or a persulphate can minimise the need for separate hot alkaline scouring [82]. In such pad–steam bleaching processes, sodium silicate was considered, until BLEACHING
  • 141. 134 PREPARATION HomeContents recently, the only effective stabiliser. Organic stabilisers have been developed to an extent that silicate can be eliminated entirely from many bleaching processes and considerably reduced for others [83]. The alternative method for minimising silicate deposition onto cloth and machinery is the use of the so-called silicate dispersants. It is strange that many of these dispersants also act as stabilisers. Magnesium salts, beneficial in long liquor bleaching, are of less importance in the saturator liquor of a pad–steam range [84], but they can improve the bleached result, once the fabric is in the steamer, by enhancing the performance of the stabiliser. In long liquors, mixtures of caustic soda and soda ash are often useful as the Na2O content can be increased without significantly raising the pH. Under pad–steam conditions no advantage is gained by using soda ash. In fact it may well be deleterious as soda ash promotes saturator liquor decomposition. In pad–steam bleaching, steam quality is of paramount importance. It should contain neither air nor rust particles. When fabric is maintained at a high temperature for more than a few minutes the steam used for heating in the scouring and bleaching stages must not contain more than 3 degC superheat. Poorly conditioned steam can lead to tendering of the fabric. High-pressure steam over 304 kPa (3 atm) must not be fed directly to steam spray pipes even if these are in the ‘wet bottom’ of the steamer. The steam should be reduced to a pressure of 101 kPa (1 atm or 15 lbf in–2 gauge) using a suitable reducing valve and then conducted through a desuperheater, fitted with suitable baffles and cooling jacket, and from which any condensate can be led to drain. The line should also be fitted with suitable filters, such as those made by Spirax Sarco, fitted with replaceable cartridges. Chemical requirements depend on fabric quality and pretreatment. The recommendations in Table 2.12 are for 100% cotton fabrics that have been caustic scoured or peroxide desized prior to bleaching [34]. Where no such pretreatment has been given it is usually necessary to increase amounts of alkali and hydrogen peroxide by 50%. The silicate may be partially or wholly replaced by an organic stabiliser. Semi-continuous processes The pad–batch process consists of padding the grey fabric with a strong solution of alkali and hydrogen peroxide and storing, in batch form, for 2–24 h depending on storage temperature. The pad–roll processes have declined due to their innate variability (section 2.5.3), but in recent years interest in cold bleaching has increased as it provides a low-energy preparation route with low capital investment, applicable to both woven and knitted fabrics. Cloth can be impregnated with the bleach solution on standard stainless steel pad mangles (or
  • 142. 135 HomeContents BLEACHING Table 2.12 Recommended conditions for continuous peroxide bleaching ___________________________________________________________________________________ J-box conveyor J-box woven or Roller-bed Pressure Jemco knitgoods rope form steamer steamer machine [42] ___________________________________________________________________________________ Magnesium sulphate heptahydrate/g l–1 0.1 0.1 0.1 0.1 0.1 Wetting agent/g l–1 2–5 2–5 2–5 0.5 5 Sodium silicate (79° Tw)/g l–1 or organic stabiliser 5–10 10–20 5–10 1.5 10 Caustic soda (100%)/g l–1 2–5 5–15 2–5 4 5 Hydrogen peroxide (35%)/g l–1 15–30 45–60 30–45 2 45 Liquor ratio 1:1 1:1 1:1 10:1 1:1 Temperature/°C 95–98 95–98 120–140 95–100 95–98 Time/min 60–120 10–30 1–2 40–60 60–90 ___________________________________________________________________________________ even jigs) but for regular production by this method an efficient open-width saturator is recommended, simply because high pick-up is difficult to achieve on grey cloth. Cold pad–batch bleaching requires: (1) control of pad bath concentration and temperature, preferably 25–35°C; (2) greater than 80% liquor pick-up; (3) fabric storage without uneven drainage or surface drying; (4) washing-off at a minimum of 95°C. A good wetting agent is essential to ensure good penetration of grey fabric by the bleaching liquor, and a good detergent is also necessary to aid fat and wax removal and development of good absorbency. A formulation for cold pad–patch bleaching is [33]: (1) sodium silicate (79° Tw) 8–12 g l–1 (2) organic stabiliser 6–10 g l–1 (3) caustic soda (100%) 8–15 g l–1 (4) hydrogen peroxide (35%) 40–50 g l–1. Persulphates can also be added, at up to 5 g l–1, to the cold pad bleaching formulation. These aid the desizing action during bleaching. After impregnation the fabric is batched, covered with a plastic sheet to prevent drying and left for a period of 15–24 h. The fabric is then washed off at
  • 143. 136 PREPARATION HomeContents 95°C in an open soaper or jig (two ends) through a solution of 1% soda ash, to which can be added soap, detergent or sequestering agent. This alkaline treatment makes a significant improvement to fabric absorbency and increases brightness by 1–2%, compared with washing-off in water only. The advantages of bleaching with hydrogen peroxide are: (1) no need for severe precleaning processes; (2) no need for exotic materials of construction, but iron and copper must not be used; (3) environmentally acceptable; no AOX even in the presence of salt; (4) decomposition products are oxygen and water; (5) excellent storage stability; (6) compatible with most dyes and FBAs; (7) gives versatile processing (batch/continuous, hot/cold, rapid/long dwell, most fibre types); (8) produces a stable white fibre with good absorbency; (9) allows route shortening by combining stages (desize with scour, scour with bleach and desize with scour and bleach). The disadvantages of bleaching with peroxide are: (1) some water is always transported; (2) sensitivity to metallic impurities; (3) multichemical baths which need control; (4) comparatively expensive. 2.8 PREPARATION OF REGENERATED CELLULOSIC FIBRES Wood pulp from certain trees can be dissolved in a solvent and can be regenerated as a fibre by extrusion of the cellulose solution into a precipitation medium. The most common solvents are caustic soda/carbon disulphide and cuprammonium hydroxide, both of which are aqueous solutions, and N-methylmorpholine-N- oxide which is an organic solvent. In the case of the aqueous solutions, the precipitation is carried out in an aqueous acid bath containing certain additives. The N-methylmorpholine-N-oxide solution is dry spun. The additives in the wet spinning bath control the nature of the product, such as normal viscose or high wet modulus (HWM) fibres [85]. Regenerated fibres are sensitive to alkali, and impurities in the precipitation bath or solvent make them prone to oxidative damage. The degree of polymerisation and resistance to alkali increase on going from normal viscose, via HWM to solvent-spun viscose. They are less variable in composition than cotton or linen and their impurities are more easily removed.
  • 144. 137 HomeContents 2.8.1 Desizing Fabrics made from these fibres are desized in exactly the same way as for cotton. Care should be exercised with oxidative desizing in terms of both alkali and oxidant concentrations, which should be no more than half of those used for cotton. Most warps contain starch-based sizes and so enzyme desizing is customary. However, carboxymethylcellulose is sometimes used, and a cold swelling process followed by hot washing with a detergent is adequate in this case. 2.8.2 Scouring [79] Soap and soda scouring is usually carried out for yarn, knitted and woven fabrics and typical processes are shown in Table 2.13. Anionic surfactants have generally replaced the true soaps as they are much easier to wash out than either soaps or nonionic detergents. The first box of the open soaper contains these additions, sequestrant is added to the second and 2 g l–1 anionic detergent added to the third. The remaining boxes are simply hot and cold rinses. On the J-box, the low-temperature steam is provide by an air-aspirator. PREPARATION OF REGENERATED CELLULOSIC FIBRES 2.8.3 Mercerising Regenerated cellulosic fibres dissolve in caustic liquor at about 6.5% by weight (15° Tw). At concentrations above and below this critical level there is much less effect. If high concentrations are used, however, it is impossible when rinsing to avoid the critical region and so causticisation rather than mercerisation is carried out on the fibres using 1.5–5% by weight caustic soda (3–12° Tw). The addition of salt reduces the risks even further. As the fibres swell less in hot caustic liquors, there is some advantage in using elevated temperatures. Table 2.13 Recommended conditions for scouring of regen- erated cellulosic fabrics ______________________________________________________ Winch Open or jet Jig soaper J-box ______________________________________________________ Anionic detergent/g l–1 1 2 5 10 Soda ash/g l–1 1 2 2 10 Temperature/°C 95 95 95 80 Time/min 30 30 30 ______________________________________________________
  • 145. 138 PREPARATION HomeContents 2.8.4 Bleaching [86,87] Most regenerated fibres are used as staple and bleached in fabric form (Table 2.14). Bleaching is only required for fluorescent whites and pastel shades. Any of the three oxidative bleaches can be used. Peracetic acid bleaching requires 3 g l–1 peracetic acid (Proxitane 4002, Solvay Interox) for 60 min at 65°C and pH 7 in the presence of sodium hexa- metaphosphate [28,51]. For winch bleaching the following chlorite bleach is recommended for 45–90 min at 90–95°C [87]: (1) sodium chlorite (80%) 0.5–1.5 g l–1 (2) bleaching auxiliary HV 0.5–1.0 g l–1 (3) sodium nitrate 1.0–2.0 g l–1 (4) sodium acetate 2.0 g l–1 (5) acetic acid to pH 4.0 Bleaching processes for normal and HWM fibres have been compared [88]. Table 2.14 Recommended conditions for bleaching of regenerated cellulosic fabrics __________________________________________________________________ Winch Roller-bed or jet Jig steamer J-box __________________________________________________________________ Sequestrant/g l–1 1 1 0.5 2 Caustic soda (100%)/g l–1 0.5–1 0.5–1 3 Sodium silicate (79° Tw)/g l–1 5 5 Organic stabiliser/g l–1 1–2 1–2 8 Hydrogen peroxide (35%)/g l–1 5 10 10–15 8.5 Temperature/°C 80 80 100 80 Time/min 60 60 4–5 45 __________________________________________________________________ 2.9 PREPARATION OF BAST FIBRES [85,89] Linen, jute, ramie and hemp are all bast fibres, forming bundles in the bast layer of plant stems. Ramie has a low content of impurities compared with the other fibres and can be processed much like cotton. The other fibres, however, have large quantities of non-cellulosic components and are usually a medium brown colour. The impurities include hemicelluloses and pectins (and their salts) together with lignin, which is responsible for the
  • 146. 139 HomeContents brown colour. In the processing of flax it is the fibre bundles that exhibit the properties associated with linen fabrics; each fibre consists of cells cemented together by the hemicelluloses. If the cells are large, as in the case of flax, this intercellular cement can be removed without undue loss in strength, although the weight loss is substantial. In jute and hemp the cells are small and removal of the hemicelluloses causes substantial weakening of the fibre. It is usual to remove the lignin from flax but not from jute [90], which makes the latter sensitive to photo- yellowing. In all cases the greater the whiteness of fibre or fabric, the greater the weight loss. 2.9.1 Jute [89] The Indian Jute Industries Research Association has described a combined hypochlorite/peroxide bleach consisting of treatment at 10:1 liquor ratio with 0.25% o.w.f. available chlorine for 30 min at pH 10.5, then treatment with the following recipe for 60 min at 70°C [90,91]: (1) hydrogen peroxide 0.25% (2) trisodium phosphate 0.25% (3) sodium metasilicate 1.0%. 2.9.2 Linen [92] Linen is the fibre obtained from the flax plant. Unbleached linen varies widely in colour because of climatic differences during growth and variable extraction (retting) of the fibre from the plant stalk by biological attack. Flax may be retted under water or on the ground, where a darker, heterogeneous fibre that is more difficult to bleach tends to be produced. Control of retting is important. An under-retted flax often has a reddish hue while a fully retted flax is almost grey in colour. Over-retting is accompanied by degradation of the cellulose. The retting process creates massive BOD/COD loads and there is a constant search for a viable chemical method confined to tanks, where the process would be more reproducible and reduction of the effluent load would be much easier. A good overview of linen wet processing is given by Sloan [92]. Linen may be processed during wet spinning (rove) or more conventionally as yarn or fabric. The preparation of linen is a compromise between the whiteness level required (often defined in quarter steps, to full white) and the weight loss that can be tolerated. The presence of woody matter (sprit) that the mechanical operations (scutching and combing) have failed to remove may be desirable in certain products or tolerated for low weight loss. In such cases, careful dye selection is PREPARATION OF BAST FIBRES
  • 147. 140 PREPARATION HomeContents necessary to give uniform coverage. High whiteness demands multiple processing and there is a danger of separating the fibre bundles (called cottonising). The risk of cottonisation is minimised by repeated mild treatments rather than one severe stage. The interplay of process stages, whiteness and weight loss has been well described by BASF [93]. Traditionally the route for producing the best white was: (1) Chlorination of the lignin, preferably with gaseous chlorine but possibly with acid hypochlorite at long liquor ratio (the solubility of chlorine in water is low). (2) Dissolution of the chlorinated lignin and hemicelluloses in alkali. This alkaline extraction stage carries some risk; the pectins are dissolved out but the heavy metal ions forming salts with them are released to impair later peroxide bleaching. (3) Improvement of the whiteness by bleaching, probably with sodium chlorite and hydrogen peroxide in sequence. With the AOX problem such routes are no longer possible but Dierkes describes combined sodium chlorite and hydrogen peroxide processes [94]. 2.10 MONITORING THE PROCESS LIQUORS [49] Whatever the preparation stage, chemicals are measured or weighed and then dissolved in water to make up the treatment liquor. During the treatment these chemicals will be consumed and, in continuous systems, it will be necessary to make further additions to maintain the concentrations in the bath. In batch processes measuring the amount of chemical consumption may also be necessary to obtain a repeatable result, for example from hypochlorite bleaching, or to be able to regenerate the bath for further use. In all cases, even with sophisticated automatic control, manual titration of the liquor must be carried out. This stems not only from the fact that automated control is not infallible – computers fail and valves stick – but also from the need to give the operative a pride in what he or she is doing. The reagent used in the titration, the titrant, is always standard solution, i.e. a solution of known molarity. Like process liquors, the impregnated or bleached substrate can also be titrated. Information about residual peroxide, for example, can often give useful data for improving the process. In this case, a known weight of substrate is immersed in the liquor in which the titration is carried out.
  • 148. 141 HomeContents 2.10.1 Titration of alkali A suitable volume of liquor, V ml (usually 10 ml), is placed in a beaker or flask and one drop of indicator, preferably bromothymol blue but, in highly coloured scour liquors, phenolphthalein, is added. Then 0.1 mol l–1 hydrochloric acid is added until the indicator changes colour. From the appropriate equations, the following equivalences can be stated: Caustic soda: 1 ml of 0.1 mol l–1 acid = 1000 × 0.004/V g l–1 NaOH (100%) Soda ash: 1 ml of 0.1 mol l–1 acid = 1000 × 0.0053/V g l–1 Na2CO3 In the case of the substrate, it is difficult to extract all the alkali and so it is often better to back-titrate, i.e. neutralise with an excess of standard acid and then titrate the unreacted acid with standard alkali. In this case the concentration of alkali on the fabric, expressed as percentage on weight of fabric (W) is given by Eqn 2.4: Concentration = f × titre × 100/W (2.4) where f is 0.004 (NaOH) or 0.0053 (Na2CO3). 2.10.2 Titration of oxidising agents In theory, any reducing agent can be used to titrate any oxidant. The method chosen here is iodometry where the oxidant liberates iodine from a potassium iodide solution and the iodine is titrated with standard sodium thiosulphate, as this technique is applicable to all three bleaching agents. The general procedure is to pipette V ml (usually 10 ml) of the liquor into a flask or beaker and add 20 ml of 150 g l–1 potassium iodide solution and 50 ml 10% sulphuric acid. The flask is then kept in the dark for 5–10 min to allow the reaction to occur. The iodine is then titrated with standard (usually 0.1 mol l–1) sodium thiosulphate (thio). X in Eqns 2.5–2.7 is the percentage strength of the oxidant, e.g. 35% for peroxide: H2 O2 : 1 ml standard thio = 0.0017 × 1000 × 100 V × X = g l −1 H2 O2 X% (2.5) NaClO2 : 1 ml standard thio = 0.00226 × 1000 × 100 V × X = g l −1 NaClO2 X% (2.6) MONITORING THE PROCESS LIQUORS
  • 149. 142 PREPARATION HomeContents NaOCl: 1 ml standard thio = 0.0035 × 1000 V = g l−1 available chlorine (2.7) When titrating a substrate, it must be removed from the acid before liberating the iodine, as the iodine would be absorbed by the substrate and thus become inaccessible for titration. The concentration of oxidant is defined by Eqn 2.4, where f is 0.0017, 0.00226 or 0.0035 as appropriate. In the case of peroxide, concentrations are usually monitored by titration with potassium permanganate and the procedure is to pipette V ml (usually 2 ml) of the liquor into a flask or beaker containing 100 ml dilute (10%) sulphuric acid solution and titrate immediately with 0.1 mol l–1 potassium permanganate. If the titre is A ml, then Eqns 2.8–2.11 hold: A/V × 4.86 = g l −1 H2 O2 35% (2.8) A/V × 4.28 = ml l −1 H2 O2 35% (2.9) A/V × 3.4 = g l −1 H2 O2 50% (2.10) A/V × 2.86 = ml l −1 H2 O2 50% (2.11) 2.11 MONITORING THE RESULTS [49,95,96] A well-prepared substrate should have: (1) a high, uniform whiteness which is stable on storage; (2) a high, uniform absorbency for aqueous solutions; (3) low chemical damage. There are national standards for measuring these values in most Western countries [95,96]. 2.11.1 Whiteness The whiteness of the substrate may be assessed by comparing it visually with a standard, or determined instrumentally using a reflectance photometer. The photometer is calibrated at 8, 16 or 32 wavelengths using a white standard (magnesium oxide, barium sulphate). An ideal standard would give 100%
  • 150. 143 HomeContents reflectance at all wavelengths. In order to assess the whiteness of a non- fluorescent cellulosic fibre or blend, the amount of light reflected in the blue region of the visible spectrum is usually measured. Whiteness of a fabric is normally assessed on multiple layers (usually between four and eight) so that the background support does not affect the reflectance. Unbleached 100% woven cotton fabrics usually have a reflectance at 460 nm of about 55%. This increases to 83–85% for fully bleached fabric. Lower reflectance, say 78–82%, is generally acceptable as a preparation for subsequent dyeing. Yarn is measured by winding the sample on to a former, and loose stock is measured in a cell with an optical glass window. The glass is also an absorber or internal reflector of the incident light and so true reflectance is reduced by 10–12%. There are no fully acceptable instruments for assessing samples whitened with FBAs. Ciba-Geigy and Bayer have considerable experience in this area, as have instrument suppliers such as Instrumental Colour Systems or other Datacolor Group members. Perspex plates, which absorb u.v., may be used to assess the bleached ground. 2.11.2 Absorbency [95] Various methods are available for testing fibre, fabric or yarn absorbency. Tests based on the time for water to be absorbed by the sample, or for a sample to sink in water, are frequently used. Well-prepared substrates should absorb a drop of water or sink in less than 1 s. In the case of fibre, a representative sample is packed into a light wire cage and the sinking time measured. The weight of water absorbed may also be measured. Yarn absorbency can be, but is rarely, assessed by the Draves test in which the sinking time of a hank is measured under standard conditions. Fabric absorbency is usually assessed using water drop absorbency times. To differentiate between samples of good absorbency, the water drop may be replaced by a drop of 50% sugar solution which increases the absorbency time by a factor of ten. The LINRA wipe tester is a more sophisticated test apparatus for fabric. In this test drops of dye solution are applied to the test paper, which is drawn under a piece of fabric. The fabric is held so that a constant load is applied. Highly absorbent fabrics rapidly absorb the dye and very short streaks are shown on the paper. Poor fabric absorbency is shown up by long dye streaks being spread on the test paper. As with the drop absorbency test, the sensitivity of the test can be modified by using dye solutions of differing viscosity. In Europe, a simple capillary rise test may be used. In this test, a thin strip of test material is MONITORING THE RESULTS
  • 151. 144 PREPARATION HomeContents suspended vertically above a solution, usually of a blue acid wool dye, with the lower edge just touching the liquor surface. The height to which the solution rises by capillary action is noted after a set time, e.g. 5 min, or alternatively the time to reach a fixed height is determined. 2.11.3 Chemical damage The degree of polymerisation of a polymer is directly proportional to the viscosity of its solution, providing all solutions are at the same concentration. In parts of Europe and the USA, this describes the basis of a test for damage in which the viscosity of a standard solution is assessed using a U-viscometer. In the UK, the reciprocal viscosity, or fluidity (F), is the basis of BS 2610, in which cuprammonium hydroxide is the solvent for the cellulosic fibre and the fluidity is measured from the flow time in special Shirley X-type viscometers. The fluidity test is more sensitive for detecting chemical damage than loss in tensile strength. In this test a weighed sample of the cellulose, finely chopped to give fibres 1–2 mm in length, is dissolved in standard cuprammonium hydroxide in a standard capillary viscometer. The quantity of cellulose is sufficient to give a 0.5% solution for cotton or a 2% solution for regenerated cellulose. The viscometer must be filled completely to exclude air and covered to prevent exposure to light, whilst controlled agitation is given over 16 h. The viscometer and contents are then brought to 20°C in a thermostatically controlled bath and the rate of flow through the capillary measured. Standard viscometers are marked so that this flow measurement is simply a timing between two points. As each viscometer is precalibrated, fluidity is then determined, from the data provided, by dividing a viscometer constant by the time recorded. This gives a direct calculation of fluidity in terms of pascals per second (Pa s–1). The principle of the test is that during dissolution no further breakdown of the cellulose molecular chain structure will occur. Long-chain material, i.e. with low damage, will provide a highly viscous solution, hence low fluidity. Unbleached cotton shows a fluidity of about two. As a general guide, bleached cotton with a fluidity of five or less is accepted commercially. Above a value of ten, chemical damage shows up as a loss in tensile strength. For regenerated cellulosic fibres, using the 2.0% solution, the high wet modulus products give similar fluidity levels to cotton, but the standard viscose has an unbleached fluidity of about ten. In some countries, particularly those in Western Europe, cupriethylenediamine (cuen) rather than cuprammonium hydroxide (cuam) is used. It is also more general to express the results as a degree of polymerisation (DP) value. A high DP
  • 152. 145 HomeContents represents undamaged and a low DP damaged cellulose. The relationship between the two systems is given by Eqn 2.12 [71,72]: DP = 2032 log 74.35 + F F − 573 (2.12) A useful adaptation of the fluidity measurement is the so-called rapid fluidity test devised by the British Textile Technology Group (the Shirley Institute). For this test, 0.125 g of cotton is dissolved in 50 ml cuprammonium hydroxide in a wide- mouthed polypropylene bottle. Six drops of 50% pyrogallol solution are added and agitation aided by addition of 12 stainless steel ball bearings, 6 mm in diameter. The bottle is mounted on a shaker and the cotton dissolves within 30 min. The solution is centrifuged and the supernatant liquor is transferred to a standard viscometer. The measured fluidity is converted to actual fluidity, using a conversion scale. This method can be used for blends of cellulosic fibres with polyester or nylon, provided the composition of the blend is known. In such cases the weight of fabric taken is that which contains 0.125 g cotton. After agitation the cellulosic fibres can be separated by centrifuging. Whilst not absolutely accurate, the technique is suitable for process control and certainly indicates any serious overbleaching situations. BS 2610 describes a modified method for linen. Viscometric methods assume that the small samples taken for testing are representative of the fabric as a whole. This is usually so and it is reasonable, therefore, to use such methods to assess scouring and/or bleaching performance. Such methods are not suited to detecting localised hydrolytic or oxidative damage to cellulose (it is difficult to distinguish between the two). Physical testing, for example tensile strength, bursting strength and abrasion resistance measurement, can be used but the results are influenced by both fabric structure and chemical damage. Tear strength is also influenced by residual, natural lubricants. Details of such tests are not given here but can be found along with many others in the British Standards Handbook No. 11 [95]. Staining tests are often useful in this area. Oxidised cellulose usually has a higher concentration of carboxy groups than the unbleached control. These resist staining with CI Direct Blue 1, or give enhanced uptake of CI Basic Blue 9 [97]. A most useful and simple test is the Harrison test for oxycellulose. The reagent is an alkaline silver nitrate solution that produces brown to black deposits of silver on the substrate where there are reducing groups (alcohols oxidised to aldehydes). Two solutions are prepared. One contains 80 g l–1 silver nitrate (A) and the other 200 g l–1 sodium hydroxide and 200 g l–1 sodium thiosulphate (B). MONITORING THE RESULTS
  • 153. 146 PREPARATION HomeContents For each gram of fabric to be tested, 2 ml of B is added to 20 ml distilled water, followed by 1 ml of A added slowly with stirring. The prepared test solution is then brought to the boil and the sample immersed. Boiling is continued, with stirring, for 5 min. Freshly cut edges can sometimes give a positive result and, when trying to distinguish mechanical (abrasion) damage from chemical damage, it is usual to slice the sample with scissors. The solution can be tested by preparing an oxidatively damaged control. This is done by spotting hypo on desized and scoured fabric and drying it in with a hot iron. Spot tests are also useful for detecting pectins (Ruthenium Red test), lignin (Phloroglucinol test), NOx (Salzmann’s reagent), heavy metal contamination and size identification on grey fabric [97]. Iron, for example, can be detected with an acidified solution of potassium thiocyanate or ferrocyanide. The latter is preferred as it is less sensitive to atmospheric contamination and the blue colour produced is insoluble in water. Size often interferes with this test and so the substrate, if fabric, should be carefully desized before testing. Copper may be detected by spotting with sodium diethyldithiocarbamate solution, but many other ions interfere and the result must be assessed carefully. When testing for copper it is advisable to spot the suspect area first with dilute nitric acid, then neutralise with dilute ammonia before spotting with the reagent. Details of these and other spot tests have been published [97]. Size identification usually involves potassium iodide solution with or without the addition of boric acid. The scheme shown in Figure 2.14, one of many such, has been suggested by Fornelli [52]. 2.11.4 Analysis of residues [58] Other materials, both inorganic and organic, may be present on the substrate before or after preparation. Their detection and estimation is usually carried out by ashing or extraction. Extraction methods may be used quantitatively on the untreated substrate but often they are not recommended on prepared goods as the experimental error (fibre loss, regain) is often of similar magnitude to the result. Furthermore, heat is always applied and there is always a danger that this will alter residues, or make them volatile. Standards usually exist for the extraction methods established in each particular country. Aqueous extracts A tared sample is Soxhlet-extracted with demineralised water, preferably in an extraction thimble to prevent fibre loss, and reweighed after suitable drying and
  • 154. 147 HomeContents MONITORING THE RESULTS Add 1–2 drops soln 1 Add 1–2 drops soln 2 Add 2 drops soln 3 + 4 drops soln 4 after 5 s, rub hard with a glass rod for 10 s Treat material for 15 min at 60–65o C in 250 ml methanol, pour off methanol and replace with water, then treat for another 15 min at 65o C. Remove fabric from treatment liquor. Add 2–3 drops soln 5 to treatment liquor Purple coloration Yellow precipitation Yellow-green coloration Brown coloration Red-brown coloration Blue coloration Presence of starch Presence of polyvinyl acetate Presence of polyvinyl alcohol (PVA) Presence of carboxymethyl cellulose (CMC) No CMC Presence of polyvinyl alcohol (PVA) No polyvinyl alcohol new piece goods (7.5 g) Standard solutions: Soln 1 1.3 g iodine + 2.4 g potassium iodide in 1 litre water Soln 2 0.13 g iodine + 2.6 g potassium iodide + 4 g boric acid in 100 ml water Soln 3 11.88 g potassium dichromate in 50 ml water + 25 ml sulphuric acid concn Soln 4 30 g sodium hydroxide in 70 ml water Soln 5 Uranyl nitrate 4% dissolved in water Figure 2.14 Scheme for size identification. (Source: [52].) conditioning. The extract is used for measuring fibre pH. In this case, the extraction flask is sealed with a suitable stopper before cooling and the result is only valid on a flask without ingress of air. Some residual water-soluble sizes may be removed, and detected with suitable reagents. The weight loss can be used as a direct measure of water-soluble material or as a measure of washing efficiency. Careful removal of the water (vacuum distillation, rotary evaporation) leaves a residue that can be examined by i.r. spectrometry. Nettles has provided a useful catalogue of the spectra for such materials but their interpretation is best left to a skilled, regular user of such techniques [5].
  • 155. 148 PREPARATION HomeContents Solvent extracts Fats and waxes are usually determined by solvent extraction of tared samples in a Soxhlet extractor. The solvents are usually chloroform for cotton and petroleum ether for polyester/cotton blends but occasionally dichloromethane is used. Careful evaporation of the solvent leaves a residue that can be examined with an infra-red spectrometer [5]. Ash content Ashing provides a measure of the inorganic residues left by careful combustion of the substrate. The amount of residue can often give indication of the presence of silicate residues, calcium salts, etc. For more specific analysis, the ash must be examined spectroscopically. Emission spectroscopy was popular for element identification: particular elements would then be quantitatively determined using atomic absorption spectrometry (a.a.). In well-equipped analytical laboratories, the initial screening is done by X-ray fluorescence (x.r.f.) and the quantitative determination is then carried out with an induction-coupled plasma spectrometer (i.c.p.). This has lowered the detection limit from parts per million to parts per billion. X.r.f. can also be carried out on the substrate itself and small areas can be investigated with an ion probe, enabling localised concentrations of cations to be identified and quantified from their X-ray spectra. The technique is weak for elements lighter than sodium. 2.12 FINAL COMMENTS The references in this review have been chosen to be freely accessible, where possible in English, via the British Library, for example. This is not always possible as the sources in German are very rich in this area and the textbooks by Rath [98], Peter and Rouette [99], and the annuals Deutscher Färberkalender [7,45,94] and Internationales Lexicon [100] must be mentioned. The last-named started out as a dictionary but has been updated with supplements as knowledge develops. Chapters 1B [98] and 7 [99] of the textbooks are particularly relevant. In quoting references from Melliand Textilberichte and Textil Praxis International E numbers and roman numerals indicate an English translation is available. The American cotton handbook [101] and Volumes 1 and 4 of the Handbook of fibre science and technology [51,85] are of interest, notably chapters A3, A4, B1 and B2 [51].
  • 156. 149 HomeContents REFERENCES 2.13 REFERENCES 1. J T Marsh, An introduction to textile bleaching (London: Chapman Hall, 1956). 2. R H Peters, Textile chemistry, Vols 1 2 (London: Elsevier, 1967). 3. E R Trotman, Textile scouring and bleaching (London: Griffin, 1968). 4. A Datyner, Surfactants in textile processing (New York: Marcel Dekker,1983). 5. J E Nettles, Handbook of chemical specialities (London: Wiley Interscience,1983). 6. U Kirner, Textile Asia, 4 (Feb 1973) 13; 6 (Nov 1975) 32. 7. P Grünig, Deutscher Färberkalender, 80 (1976) 48. 8. J F Lueck, Am. Dyestuff Rep., 68 (Nov 1979) 49. 9. P Wurster, Textil Praxis, 47 (1992) 960, VI. 10. V Olip, Melliand Textilber., 73 (1992), 819, E377. 11. V A Shenai, Technology of textile processing, III Technology of bleaching (Bombay: Sevak Publications, 1976). 12. BTRA Silver jubilee monograph: Bleaching and mercerising (Bombay: BTRA, 1985). 13. W Sebb, Textil Praxis, 43 (1988), 841, XII. 14. M Petrick, Internat. Text. Bull., 34, No. 3 (1988) 58. 15. K H Gregor, Melliand Textilber., 71 (1990), 976, E435. 16. M Pittroff and K H Gregor, Melliand Textilber., 73 (1992), 526, E244. 17. R H Tieckelmann et al., AATCC International Conference and Exhibition (Oct 1992), 175. 18. J Park and J Shore, J.S.D.C., 100 (1984) 383. 19. E J Newton, J.S.D.C., 109 (1993) 138. 20. C Duckworth, Ed., Engineering in textile coloration (Bradford, Dyers Company Publications Trust, 1983). 21. M Paterson, Rev. Prog. Coloration, 4 (1973) 80. 22. D H Wyles, Rev. Prog. Coloration, 9 (1978) 35. 23. H U von der Eltz, Textil Praxis, 43 (May 1988) XVII; 43 (July 1988) XXI, XXIV; 47 (Mar 1992) XX. 24. R C Cheetham, Dyeing fibre blends (London: Van Nostrand,1966). 25. A Winch, Text. Res. J., 50 (Feb 1980) 64. 26. J Park, A practical introduction to yarn dyeing (Bradford: SDC, 1983). 27. B A Evans, Text. Chem. Colorist, 13 (Nov 1981) 254. 28. Solvay Interox, Bleaching Manual (1977). 29. C Garrett, J.S.D.C., 71 (1955) 830; 80 (1964) 117. 30. G R Turner, Text. Chem. Colorist, 20 (Oct 1988) 25. 31. P S Collishaw and D T Parkes, J.S.D.C., 105 (1989) 201. 32. M Stuhlmiller and H Lehmann, Textil Praxis, 37 (1982), 524; XIII. 33. K Dickinson and W S Hickman, J.S.D.C., 101 (1985), 283. 34. Solvay Interox, Brochure AO.2.5 Continuous preparation of cellulosic and blend fibres (1986). 35. Text. Chem. Colorist, 13 (Nov 1981). Special issue on current trends in textile preparation. 36. B D Bähr et al., Textil Praxis, 46 (1991) 973; 47 (1992) 1041. 37. Anon., Internat. Text. Bull., 36, No.3 (1990) 83. 38 Anon., Melliand Textilber., 71 (1990) 398. 39. Anon., Melliand Textilber., 72 (1991) 775, E319. 40. H C Paulsen, Internat. Text. Bull., 36, No. 3 (1990), 67. 41. B C Burdett and J G Roberts, J.S.D.C., 101 (1985) 53. 42. B L McConnell, W Prager and B K Easton, Text. Chem. Colorist, 14 (Apr 1982) 76.
  • 157. 150 PREPARATION HomeContents 43. M H Rowe, AATCC National Technical Conference (Oct 1977) 64. 44. U Kirner, Textil Praxis, 27 (1972) 543. 45. C A Theusink, Deutscher Färberkalender, 81 (1977) 102. 46. C Tischbein, J.S.D.C., 105 (1989) 431. 47. K Adrian, Textil Praxis, 31 (1976) 785. 48. G Knödler, Textilveredlung, 21 (1986) 163. 49. Solvay Interox: A bleachers handbook (1993). 50. I Holme, Dyer, 158 (9 Dec 1977) 626. 51. M Lewin and S B Sello, Eds, Handbook of fibre science and technology, Vol.1 Chemical processing (New York: Marcel Dekker, 1983). 52. S Fornelli, Textil Praxis, 45 (1990) 1178, VII. 53. O Deschler and G Schmidt, Textil Praxis, 36 (1981) 1331, IX. 54. R Freytag, Textilveredlung, 1 (1966) 291. 55. M H Rowe, Text. Chem. Colorist, 10 (Oct 1978) 215. 56. L A Sitver, AATCC National Technical Conference (Oct 1977) 71. 57. D J Campbell, Am. Dyestuff Rep., 33 (1944) 293. 58. T E Bell and N J Stalter, Am. Dyestuff Rep., 41 (1952) 110. 59. G Rösch, Textil Praxis, 43 (Jan 1988) 61, II; (Mar 1988) 264, VII; (Apr 1988) 384, VII; (May 1988) 515, XIX; (June 1988) 615; (Aug 1988) 847, XX; 44 (Jan 1989) 38, XVIII. 60. C Duckworth and L M Wrennall, J.S.D.C., 93 (1977) 407. 61. D Bechter and G Kunz, Textil Praxis, 34 (1979) 965, 1369. 62. P F Greenwood, J.S.D.C., 103 (1987) 342. 63. A Oparin and S Rogowin, Melliand Textilber., 11 (1930) 944. 64. J Dornhagen et al., Textil Praxis, 43 (1988) 396. 65. R L Derry, J.S.D.C., 71 (1955) 884. 66. J A Epstein and M Lewin, J Polym. Sci., 58 (1962), 991, 1023. 67. Benninger Engineering, Dyer, 157 (1976) 558. 68. H Hefti, Text. Res. J., 30 (1960) 861. 69. L Chesner and R A Leigh, Textil-Rundschau, 20 (1965) 217. 70. J K Skelly, J.S.D.C., 76 (1960) 469. 71. P Ney, Textil Praxis, 29 (1974) 1392, 1552. 72. P Ney, Melliand Textilber., 63 (1982) 443. 73. V Neveling and W Sebb, Textilbetrieb (Dec 1975), 37; (June 1976), 41. 74. D M Cates and A M M Taher, Text. Chem. Colorist, 7 (1975), 220. 75. E Naujoks, Textil Praxis, 24 (1969) 167, E85. 76. P Wurster, Textilveredlung, 20 (1985) 187. 77. W A Millsaps, AATCC National Technical Conference (Oct 1977) 72. 78. A Kling, Textil Praxis, 39 (1984) 187, 686. 79. D Bechter and A Roth, Textil Praxis, 45 (1990) 500, VII. 80. I Solajacic, A M Grancaric and M Kusari, Tekstil, 27 (1978) 27. 81. K Dickinson and D Heathcote, J.S.D.C., 88 (1972) 137. 82. W S Hickman, Dyer, 159 (1978) 600. 83. W S Hickman and H Andrianjafy, J.S.D.C., 99 (1983) 86. 84. J Kymel and O Merk, Textil, 12 (June 1957) 226. 85. M Lewin and S B Sello, Eds, Handbook of fibre science and technology, Vol. IV Fibre chemistry, (New York: Marcel Dekker, 1983). 86. C E Harrison and I H Welch, AATCC National Technical Conference (Oct 1972) 307. 87. Courtaulds, Viscose technical information manual – Fibro.
  • 158. 151 HomeContents REFERENCES 88. F Kocevar and M A Talovic, Melliand Textilber., 54 (1973) 1064. 89. R R Mukherjee and M A Radhakrishnan, Text. Progress, (1972) 4. 90. M A Guha Roy et al., J. Text. Inst., 20 (Jan 1988) 108. 91. IJIRA Monograph, Chemical processing of jute fabrics for decorative end-uses, Part 1 Bleaching (1979). 92. F R H Sloan, Rev. Prog. Coloration, 5 (1974) 12. 93. BASF Manual, Cellulosic fibres B375e (1979). 94. G Dierkes, Deutscher Färberkalender, 34 (1986) 39. 95. British Standards Institution, Handbook No. 11 (1974). 96. AATCC, Technical manual (annual). 97. W Garner, Textile laboratory manual, 3rd Edn (London: Heywood Books,1966). 98. H Rath, Lehrbuch der Textilchemie, 3rd Edn (Berlin: Springer-Verlag,1972). 99. M Peter and K H Rouette, Grundlagen der Textilveredlung, 13th Edn (Frankfurt: Deutscher Fachverlag,1989). 100. C H Fischer-Bobsien, Internationales Lexikon Textilveredlung und Grenzgebiet (annual). 101. D S Hamby, Ed., The American cotton handbook, 3rd Edn, Vol 2 (New York: Interscience Publishers,1965).
  • 159. 152 DYEING WITH DIRECT DYES HomeContents 152 CHAPTER 3 Dyeing with direct dyes John Shore 3.1 INTRODUCTION Direct dyes are defined as anionic dyes with substantivity for cellulosic fibres, normally applied from an aqueous dyebath containing an electrolyte. Their most attractive feature is the essential simplicity of the dyeing process, but a separate aftertreatment to enhance wet fastness has been necessary for most direct dyeings since the end of the last century [1]. The two most significant non-textile outlets for direct dyes are the batchwise dyeing of leather and the continuous coloration of paper. The standard of wet fastness, particularly to washing, of direct dyeings, even when given a conventional cationic aftertreatment, does not meet the more demanding end uses in cellulosic apparel and furnishing materials. Consequently, direct dyes have been replaced to a great extent by reactive dyes, which have better wet fastness and exceptional brightness in many hues. Nevertheless, there are still many applications in the textile industry for goods dyed with direct dyes, particularly where a high standard of wet fastness is not required. Resin finishing after dyeing produces a notable improvement in wet fastness, especially on regenerated cellulosic fabrics. The development of specialised aftertreating agents and crosslinking reactants for use with selected direct dyes of high light fastness has reached new levels of sophistication, enabling direct dyes to compete more effectively with reactive dyes in meeting severe wet fastness requirements [2–4]. Direct dyes are used in low-priced viscose or blended curtain fabrics, furnishings and carpets, where good light fastness and moderate fastness to washing are usually adequate. Cheap cotton dressing gowns and bedspreads that may be washed infrequently, viscose ribbons and linings, flannelettes and winceyettes for baby-wear, as well as dischargeable ground shades for low- quality prints are often dyed with direct dyes. They may also be used in cellulosic
  • 160. 153 HomeContents or blended fabrics for casual wear and rainwear, provided that an appropriate durable press or water-repellent finish is given. 3.2 STRUCTURE AND PROPERTIES OF DIRECT DYES FOR CELLULOSE More than 75% of all direct dyes are unmetallised azo structures and these are represented in every hue sector [5]. In contrast to all other application ranges, the great majority of them are disazo or polyazo types, the former predominating in the brighter yellow to blue sectors and the latter in the duller greens, brown, greys and blacks. The copper-complex direct dyes are mostly duller violets, navy blues and blacks derived from the disazo and polyazo subclasses, mainly used in maroon, brown, grey, navy and black recipes. Stilbene and thiazole dyes, the only non-azo chromogens in the yellow to red sector, bear certain similarities to the disazo and polyazo structures. The dioxazine and phthalocyanine chromogens are represented in bright blue direct dyes of high light fastness. 3.2.1 Chemical classification Approximately 50% of all direct dyes are disazo structures, a typical example being the symmetrical dianisidine derivative CI Direct Blue 1 (Figure 3.1). –O3S SO3Na NH3 OH N N N N OCH3H3CO OH NH3 SO3 – SO3Na Figure 3.1 A major application for copper complexes is in the prior metallisation or aftertreatment of direct dyes containing at least one o,o′-dihydroxyazo or o- methoxy-o′-hydroxyazo chromophoric system. The best-known example is CI Direct Blue 76, a greenish blue copper complex (Figure 3.2) derived from CI Direct Blue 1 by metallisation with cuprammonium sulphate in the presence of an alkanolamine. Triazine ring structures are used in the manufacture of certain azo dyes, particularly those containing two separate chromophoric systems. In the resulting product each chromogen contributes its own absorption characteristics; by combining yellow and blue chromogens, green dyes are formed that are much STRUCTURE AND PROPERTIES OF DIRECT DYES FOR CELLULOSE
  • 161. 154 DYEING WITH DIRECT DYES HomeContents Figure 3.4 N S SO3Na H3C N S NH2 brighter than conventional polyazo dyes. A typical example is CI Direct Green 28 (Figure 3.3) in which cyanuric chloride is used to link a non-substantive anthraquinone derivative to a substantive phenylazosalicylic acid intermediate. The characteristic chromophore of azophenylthiazole direct dyes is the thiazole ring itself, normally forming part of a 2-phenylbenzothiazole grouping. Most are yellow direct dyes such as CI Direct Yellow 59 (Figure 3.4), which contains two of the thiazole rings that enhance their substantivity for cellulose. Stilbene direct dyes are mainly yellow, orange or brown. They are mixtures of indeterminate constitution resembling polyazo direct dyes in their application properties. They result from the alkaline self-condensation of 4-nitrotoluene-2- sulphonic acid or its initial condensation product. The characteristic chromophores are azo- or azoxy-stilbene groupings and these dyes are mostly non-dischargeable. Azostilbene dyes of more precise constitution are prepared by tetrazotisation and coupling of 4,4′-diamino-2,2′-stilbenedisulphonic acid, such as CI Direct Yellow 12 (Figure 3.5). O O NH3 SO3 – NH SO3Na NH N N N NH NH N N COONa OH Figure 3.3 Figure 3.2 –O3S SO3Na NH3 O N N N N O NH3 SO3 – SO3Na CuII O O CuII
  • 162. 155 HomeContents CH3CH2O N N CH CH N N OCH2CH3 SO3Na NaO3S Figure 3.5 Phthalocyanine direct dyes are water-soluble sodium salts of sulphonated copper phthalocyanine, e.g. the disulphonate with sulpho groups in the 3-positions (Figure 3.7). They have poor absorption properties so that deep dyeings are virtually unobtainable. Dyeing has to be done at about 95°C in order to facilitate absorption and this precludes their application in low-temperature dyeing processes such as pad dyeing. Problems of poor reproducibility are encountered in exhaust dyeing. They give brilliant turquoise blue colours of high light fastness but poor wet fastness unless resin finished. They are applicable to paper where wet fastness is not as significant as on cotton and viscose. N N N N Cu←→ NaO3S SO3Na Figure 3.7 The triphenodioxazine ring system is the basis of some important blue direct dyes such as CI Direct Blue 106 (Figure 3.6). The tendency in recent years for the technically excellent but less cost-effective anthraquinone reactive dyes to be replaced by alternative bright blue derivatives of phthalocyanine, triphenodioxazine and formazan chromogens has a spin-off effect in yielding non-reactive precursors that are directly applicable as direct dyes. N H O N N O H N SO3NaNaO3S Figure 3.6 STRUCTURE AND PROPERTIES OF DIRECT DYES FOR CELLULOSE
  • 163. 156 DYEING WITH DIRECT DYES HomeContents 3.2.2 Replacements for benzidine-derived dyes Congo red (CI Direct Red 28), the first direct dye marketed, was produced from benzidine and naphthionic acid (Figure 3.8). It soon lost its importance for dyeing cotton because of its extreme sensitivity to acids, and was later used mainly as an acid indicator. Many direct dyes of former importance made from benzidine have been withdrawn from manufacture on account of the occurrence of carcinoma of the bladder in operatives, which is known to be caused by benzidine and certain derivatives. Epidemiological studies carried out in the early 1950s revealed this increased incidence of bladder cancer after exposure to benzidine, but it was not until the 1970s that the major dyemakers agreed to phase out the manufacture of benzidine-derived dyes. An analytical study of more than 40 direct dyes requiring the use of benzidine in their manufacture demonstrated that all contained 2 mg kg–1 benzidine or more as an impurity and three of them contained as much as 50–70 mg kg–1 [6]. NH3 SO3 – N N N N NH3 SO3 – Figure 3.8 An intensive search was undertaken into alternatives produced from less hazardous intermediates [7], principally the following: (1) The substitution of existing benzidine derivatives by analogous dyes in which benzidine is replaced by a structurally similar but less hazardous diamine such as o- or m-tolidine, or dianisidine; however, these diamines are also suspected of having carcinogenic activity and are controlled substances. (2) The use of new diamines, including 4,4′-diaminobenzanilide, 4,4′-diamino- diphenylbenzaldehyde, 4,4′-diaminodiphenyl ether, 4,4′-diaminodiphenyl- sulphone and 1,5-diaminonaphthalenes. Apart from the brighter symmetrical disazo direct dyes such as Congo red, many traditional brown or black polyazo dyes for application to cellulosic fibres and leather, such as CI Direct Black 38 (Figure 3.9), relied on the extremely versatile intermediate benzidine. The two diazonium groups can be coupled selectively in a stepwise manner to produce many useful products.
  • 164. 157 HomeContents N N N N NH2 H2N NH2 NaO3S SO3Na N N OH Figure 3.9 However, benzidine was found to be carcinogenic and its withdrawal by the major dyestuff manufacturers has led to the discontinuance of dyes such as Black 38 in favour of products such as CI Direct Black 166 (Figure 3.10) in which 4,4′- diaminobenzanilide is used as a benzidine replacement. Another novel intermediate synthesised for this purpose is 7-amino-3-(4′- aminophenyl)quinoline that has been used to prepare various disazo and trisazo derivatives. The differential reactivity of this tetrazonium salt is helpful in this regard, the diazonium cation on the phenyl ring being the more reactive, but unfortunately the novel dyes are deficient in tinctorial power and light fastness [8]. A more general objective has been to identify and utilise precursor intermediates that have been shown to be non-genotoxic. Increasing use is being made of the published results of the mutagenicity and carcinogenicity testing of azo dyes and their intermediates in this field. It is clearly helpful in the design of non-carcinogenic azo dyes to ensure that the intermediates are non-mutagenic and to take into account the potential genotoxicity of the metabolites resulting from the reductive cleavage of the azo linkages. Following this approach, the replacement of benzidine by 5,5′-diamino-2,2′-bipyridine in four well-known carcinogenic azo structures was examined. It was shown that this novel component was much less genotoxic than benzidine itself and the derived dyes were also less genotoxic than their older analogues made from benzidine [9]. The effect on mutagenicity caused by incorporating an alkoxy substituent into the structure of a water-soluble disazo dye was investigated. The results indicated N N N N NH2 H2N NH2 NaO3S SO3Na N N OH NHCO Figure 3.10 STRUCTURE AND PROPERTIES OF DIRECT DYES FOR CELLULOSE
  • 165. 158 DYEING WITH DIRECT DYES HomeContents that, although bulky alkoxy groups were useful in lowering the mutagenicity of certain analogues of CI Direct Black 17, the decrease observed was less than that noted for a series of monoazo disperse dyes [10]. 3.2.3 Reactant-fixable direct dyes One of the most significant breakthroughs in the field of direct dyes in recent years has been the introduction by Sandoz of Indosol SF dyes and a series of Indosol fixing agents for use with them [2–4,11–15]. The original range of Indosol SF dyes comprised twelve members, all copper complexes selected from the existing Solar range for good light fastness when aftertreated with the preferred agents. Few of these dye structures have been disclosed but CI Direct Violet 66 (Figure 3.11), marketed as Indosol Violet SF-B, gives an indication of the molecular characteristics represented. Figure 3.11 O NN H2NO2S O CuII NaO3S O N H N N SO2NH2 OCuII SO3Na The original range of Indosol SF dyes was strong in the blue and grey sectors but somewhat deficient in the yellow to violet series. Novel reactive yellow and red dyes were later introduced to supplement the range, slightly blurring the distinction between the reactive and reactant-fixable classes. The specific aftertreating agents used with Indosol SF dyes and their methods of application to cellulosic fibres and blends are discussed later (section 3.7.7). 3.2.4 Photochromism of direct dyeings Certain direct dyes, principally sulphonated copper phthalocyanines, exhibit a photochromic change on cellulosic fibres, the hue changing from bright turquoise to violet or reddish blue on prolonged exposure to sunlight or u.v. radiation. The hue reverts gradually to the original turquoise when the illuminant is withdrawn. This effect is accentuated by the presence of dye-fixing agents or crease-resist finishes, particularly if the dyed material is stored in an
  • 166. 159 HomeContents acidic condition. The type of crease-resist finish plays a significant part, the order of decreasing effect being: urea–formaldehyde cyclic alkyleneurea reactants melamine–formaldehyde. The degree of photochromic change is related to the intensity of the u.v. radiation and the ambient humidity. The drier the atmosphere, the slower the reversion to the original turquoise in the absence of the illuminant. Thus products that lower the moisture regain, e.g. crease-resist finishes, greatly increase the tendency to show photochromic change. The photochromic behaviour of copper phthalocyanine direct dyes appears to follow a redox mechanism. When dyeings of this type on cellulosic fibres are subjected to a vatting procedure with sodium hydroxide and sodium dithionite at 60°C, the hue changes rapidly from turquoise blue to violet. The dyeing reverts to its original hue on rinsing in water followed by air oxidation, thus producing similar changes to those taking place on illumination. It is interesting to note that the unsulphonated copper phthalocyanine pigments do not exhibit this redox behaviour. 3.2.5 Aggregation of direct dyes in solution Dyes often exist in aqueous solution as aggregates of several ions or molecules rather than as individual molecules. It is not necessary to assume that all component molecules in a dye aggregate are in the same state of ionisation. Dye aggregates should be envisaged as relatively amorphous in composition with zones of more or less polar character distributed within them, although there will still be a tendency for the individual molecules to become oriented with their ionised groupings towards the aqueous phase. Aggregation is promoted by a high ratio of relative molecular mass to ionic group content, that is, by a high value for the equivalent mass per sulphonic acid substituent in the case of sulphonated anionic dyes. Multisulphonated dye anions may be stabilised as dimers in which the two components adopt a planar arrangement with the sulpho groups located if possible at opposite ends of the dimer. Dimers are formed before larger aggregates, growing further by accretion of more dye anions to form lamellar micelles in which the dye units are stacked like cards in a pack [16]. Planar chromophores, especially the sulphonated phthalocyanines, are particularly prone to such stacking. Most measurements of aggregation number (the average number of molecules/ions per aggregate) have been carried out on solutions of direct dyes, since these dyes aggregate more readily than most of the other classes of water- soluble dyes. Regrettably, aggregation numbers determined for the same dye STRUCTURE AND PROPERTIES OF DIRECT DYES FOR CELLULOSE
  • 167. 160 DYEING WITH DIRECT DYES HomeContents under the same conditions using different methods of measurement are seldom consistent. Most investigators agree, however, that such solutions contain a mixture of aggregates of various sizes in dynamic equilibrium. Individual ions, dimers or trimers are removed by adsorption onto the fibre surface during dyeing and larger aggregates break down to maintain a similar overall distribution of aggregate sizes. The extent of aggregation of a direct dye decreases with increasing temperature and increases with increasing concentration of added electrolyte or of the dye itself. Although many direct dyes are highly aggregated at room temperature, the degree of aggregation is often negligible under normal dyeing conditions at the boil even in the presence of an electrolyte [17]. Although quantitative agreement between the aggregation numbers obtained using different techniques of measurement is poor, meaningful conclusions can be drawn from them regarding the relative tendencies to aggregate of different dye structures. Under given conditions of temperature and electrolyte concentration, CI Direct Orange 26 (Figure 3.12) is much more highly aggregated than CI Direct Yellow 12 (Figure 3.5). Both are disulphonated disazo dyes, but presumably the two phenolic groups and especially the urea residue in the former dye provide much greater scope for intermolecular hydrogen bonding than the etherified phenolic groups in the latter. Figure 3.12 N N NaO3S OH N OH SO3Na N N H C N H O CI Direct Red 28 (Figure 3.8) is highly aggregated, no doubt because of electrostatic intermolecular attraction between matching pairs of protonated primary amino groups and sulphonate anions of the naphthionic acid residues. CI Direct Blue 1 (Figure 3.1) is considerably less aggregated because the charges on the protonated amino groups are satisfied by intramolecular association with their neighbouring sulphonate groups, and the dye anion thus has a net negative charge of two. 3.2.6 Decomposition of direct dyes during dyeing If a direct dyeing is unduly prolonged, either to improve the levelness or to attain an exact colour match to standard, decomposition of a minor proportion of the
  • 168. 161 HomeContents dye present may occur. The rate of decomposition is normally much more rapid in the dyebath phase then on the fibre and the onset of degradation can sometimes be observed as a change in colour of the exhaust liquor. Many direct dyes show instability of this nature if applied at temperatures above the boil. A frequent cause of the problem is reduction of the azo linkages; the decomposition may be accelerated in the presence of viscose, which has a reducing action under alkaline dyeing conditions [18]. Buffering to pH 6 with ammonium sulphate confers a stabilising influence when dyes of lower stability have to be used on viscose. Mild oxidants such as sodium m-nitrobenzenesulphonate are occasionally useful, and more powerful inorganic oxidants, such as potassium chlorate or dichromate, can be most effective if used with care. An alternative approach is to use sodium perborate but it is necessary to exclude copper, which would cause catalytic degradation of cellulose, and the dyes used must be unaffected by sodium perborate. Direct dyes with exposed azo groups free from electron-withdrawing o- substituents are particularly prone to reductive decomposition, CI Direct Yellow 12 (Figure 3.5) being a typical example. Some dyes with protonated o-aminoazo groups, such as CI Direct Red 28 (Figure 3.8), are also reduction-sensitive [19]. In the relatively unstable trisazo structure of CI Direct Green 33 (Figure 3.13), the most vulnerable of the three azo groups is the unprotected central linkage. N N N N N OHSO3Na N OCH2CH3H3C H3C NHCOCH3 SO3NaNaO3S NaO3S Figure 3.13 Dyes containing azo groups protected from electrophilic attack by o,o′- substituents, especially o-sulphonate groups, are generally relatively resistant to reductive breakdown during dyeing. Azothiazole dyes such as CI Direct Yellow 59 (Figure 3.4) and especially copper-complex structures such as CI Direct Violet 66 (Figure 3.11), in which both azo groups are fully protected as the metal complex, show excellent stability in high-temperature conditions [20]. Direct dyes of the diphenylurea type, such as CI Direct Orange 26 (Figure 3.12), are particularly prone to decomposition in high-temperature dyeing because molecular breakdown can occur by hydrolysis [21] as well as by the reductive STRUCTURE AND PROPERTIES OF DIRECT DYES FOR CELLULOSE
  • 169. 162 DYEING WITH DIRECT DYES HomeContents mechanism under alkaline conditions. Thus the ureido linkage may be broken by hydrolysis to give monoazo dye fragments, or the azo groups can be reduced and naphthylamine breakdown products formed. 3.2.7 Dischargeability of direct dyeings As already noted, more than 75% of all direct dyes are unmetallised azo structures and without aftertreatment may show good dischargeability. They are suitable for dyeing fabrics that are to be printed with a white or coloured discharge; the lack of wet fastness is a disadvantage for many end uses but this can be overcome by subjecting the discharge print to a resin-finishing process. Diazotised and developed direct dyeings (section 3.7.1) generally exhibit very good dischargeability, providing excess developer is removed. With fabrics intended for discharge printing, residual sodium nitrite and acid must be removed after diazotisation, or undesirable by-products may be formed during coupling. During discharge printing the reducing agent (sodium formaldehyde- sulphoxylate) reduces the azo groups in the correctly developed dyes but not in the by-products, resulting in an unsatisfactory discharge. Premetallised direct dyeings and those aftertreated with copper salts are generally less readily dischargeable than those dyeings that do not contain copper. This is a limiting factor in their use as dyed grounds for discharge printing. The maximum discharge effect is obtained by dyeing, discharging and then after-coppering, but difficulties due to inadequate wet fastness arise when washing-off the uncoppered discharge print. Printers generally prefer to after- copper the dyeing before discharge printing. Non-azo direct dyes, including stilbene derivatives, sulphonated copper phthalocyanines and triphenodioxazine dyes, are not suitable as grounds for discharge printing. 3.3 DYE STRUCTURE AND DYEABILITY OF CELLULOSIC FIBRES The degree of sulphonation is decisive for the dissolution of direct dyes in water and the tendency of such dyes to form aggregates (section 3.2.5). In general, the lower the equivalent mass (i.e. the ratio of relative molecular mass to ionic group content), the higher will be the aqueous solubility of the dye. Exceptions from this simple rule can arise, however, if a direct dye structure contains protonated amino groups (as in Figures 3.1 or 3.8, for example). As already noted, the converse of this general trend is that dye molecules of higher equivalent mass tend to aggregate more readily because of the greater
  • 170. 163 HomeContents opportunity for hydrophobic interaction. Such dyes in the presence of a cellulosic substrate will tend to escape more readily from the aqueous dyebath by adsorption onto the fibre surface. It is reasonable to assume that the dye molecules will become adsorbed as far as possible in such a way that the hydrophilic sulphonate ionic groups are directed outwards towards the aqueous phase. 3.3.1 Dye structure and substantivity for cellulose Just as addition of electrolyte to a solution of direct dye tends to lower the electrostatic repulsion between the negatively charged dye anions and promote aggregation, the effect of the presence of an inorganic salt when dyeing cellulose is to overcome the long-range forces of repulsion between the dye anions and the negatively charged fibre surface. The closer approach then allows hydrogen bonding and other short-range attractive forces to operate between the dye molecules and the glucoside units of the fibrous polymer. Hydrogen bonding between the hydroxy groups of cellulose and centres of electronegativity (nitrogen, oxygen and sulphur atoms) in the dye molecule, especially those substituted with hydrogen atoms (as in =N–NH–, –NH2, –CONH–, –OH and –SH), is widely acknowledged to contribute to adsorption and retention of the dye molecules. Stereochemical studies have demonstrated that hydrogen bonding is possible for almost any cellulosic hydroxy group in the amorphous regions of the fibre. The more hydrogen bonds that a dye can form with the glucosidic polymer, the more readily it can compete with and rupture the fibre–fibre hydrogen bonds in order to penetrate more deeply into the amorphous structure of the polymer. Many direct dyes with high affinity for cellulose are disazo or trisazo structures in which the azo groups are located in para positions relative to one another, so that the longest conjugated chain through the azo-linked aryl nuclei tends to be as near linear as practicable. This is facilitated in dyes derived from intermediates contributing a 1,4-phenylene or -naphthylene (Figure 3.13), 4,4′- biphenylene (Figure 3.1 or 3.8) or 2,6-naphthylene (Figure 3.12) residue. A methine group oriented para to an azo group through an intervening aryl nucleus (Figure 3.5) boosts substantivity even more than does an amide group in a similar position [22]. The significance of conjugation as a contributor to the substantivity of dyes for cellulose is not always easy to distinguish from the effect of the degree of linearity of the molecule. The molecules of almost all direct dyes possess flexible chains of aryl nuclei linked by azo or other groups. Such structures can DYE STRUCTURE AND DYEABILITY OF CELLULOSIC FIBRES
  • 171. 164 DYEING WITH DIRECT DYES HomeContents readily adopt a near-linear spatial conformation, as far as the angular constraints of the participating bonds will allow. Hence azo groups attached to central naphthylene residues are frequently oriented in 1,4- or 2,6-positions relative to one another and typical central diamines give linear dyes on tetrazotisation. The ability to adopt an extended configuration has long been recognised [23] to be a desirable feature of substantive dyes. A further refinement of the concept that linear conjugated aryl–azo–aryl–azo–aryl sequences and analogous systems represented the preferred design for highly substantive molecules was the stipulation that the aryl nuclei must be able to adopt a coplanar conformation [24]. This favours the adsorption of one dye molecule on to another during the formation of laminar aggregates, as well as the multi-point adsorption of a dye molecule on to two or more glucoside units of a polymer segment via hydrogen bonding. 3.3.2 Ability of direct dyes to cover neps in cotton fabrics Pale flecks in dyed cotton fabrics can be caused by immature or dead cotton. Clumps or neps of immature fibres tend to exhibit poor dyeability and the flattening of such neps may form small, highly reflective flecks. Poor dye penetration may leave undyed areas if the neps of loosely attached immature fibres change position after dyeing. The dyeability of immature cotton can be improved by mercerisation but the lack of secondary cell wall development in dead cotton makes such treatment ineffective [25]. Direct dyes vary considerably in their ability to cover dead or immature cotton, but by careful selection a sufficient range can be found suitable for this purpose. Cotton fabrics containing both immature fibre neps and process neps composed of damaged mature fibres were given various mercerising treatments. After dyeing with ten direct dyes, fully mercerised fabrics showed a 95% increase in visible coverage of neps, but causticised fabrics only an 85% increase in coverage. After liquid ammonia treatment, significant improvement in nep coverage occurred with only five of the ten dyes [26]. The differences between direct dyes in their behaviour towards nep coverage are most evident with those neps composed of immature fibres with some secondary wall development [27]. Direct dyes containing more than one amino or amide group in their structure were found most likely to achieve relatively good coverage of neps [28]. Although good coverage bears no obvious relationship to molecular size or shape, aggregation and migration properties of direct dyes do appear to be related to nep coverage [27].
  • 172. 165 HomeContents 3.3.3 Effect of mercerisation and liquid ammonia treatment on direct dyeing Comprehensive reviews of caustic soda mercerisation and its effect on direct dye uptake are available [29,30] and several studies of possible mechanisms have been published. Scoured-only or scoured and bleached cotton yarns were slack mercerised, restretched and then dyed with a disazo direct blue at 70°C. The exhaustion and rate of dyeing of the mercerised yarns were appreciably higher than those of the scoured yarns but these differences decreased as the degree of restretching was increased. A hypochlorite bleach after scouring also decreased the difference in dye uptake between scoured and mercerised yarns [31]. The relationship between fibre porosity and direct dye uptake was examined by preswelling with caustic soda or zinc chloride, followed by measurement of pore size distribution and porosity. Average dye savings of 30% were demonstrated by treatment of cotton fabrics with 20% sodium hydroxide solution for 1 min at 20°C, washing, neutralisation and dyeing with three typical direct dyes. The improved colour yield was attributed partly to increased dye uptake and partly to the optical effect of fibre swelling and internal light scattering influenced by the change in total pore volume [32]. Dyeings of CI Direct Red 81 (Figure 3.14) and Blue 1 (Figure 3.1) on bleached cotton fabrics that had been mercerised in various ways also provided evidence that the dye saving is related to the reduction in diffuse light scattering and depends on the method of pretreatment given. The reduced light scattering occurs at the interface between the fibre secondary wall and the cell lumen [33]. Scoured and mercerised samples of cotton were compared with viscose staple and grey cotton for steam sorption and water retention to characterise their physical structure. Sorption measurements with sugars and dextrans of graduated sizes were used to determine changes in pore structure caused by mercerising. By correlating the pore structure with the uptake of CI Direct Blue 1 (Figure 3.1), it was deduced that transition pores in the cellulosic fibres with a diameter of about 20–60 nm are responsible for dye sorption [34]. Characteristics of the pore structure of cotton and viscose in the water-swollen state enables new volume terms to be determined for calculation of thermodynamic substantivity [35]. Substantivity parameters for CI Direct Blue 1 NaO3S N N N OH N NHCONaO3S Figure 3.14 DYE STRUCTURE AND DYEABILITY OF CELLULOSIC FIBRES
  • 173. 166 DYEING WITH DIRECT DYES HomeContents on cotton under various application conditions after hot mercerising at 60 and 90°C have been given [36]. Warm mercerisation of grey cotton fabrics at temperatures below 50°C resulted in little difference in adsorption of a symmetrical disazo direct blue, although treatment above 50°C did produce a measurable increase [37]. Grey or bleached cotton and cotton/polynosic knitted fabrics showed decreased uptake of direct dyes with increase in mercerising temperature up to 80°C, but higher fabric tension tended to enhance dye uptake. Shade variations between the centre and selvedges of dyed knitgoods may be attributable to differences in tension during mercerisation [38]. Liquid ammonia pretreatment is a highly effective and well-controlled alternative to caustic soda mercerisation, but the high capital cost of the necessary equipment for recovery and reuse of the ammonia as well as the application step limits the adoption of this sophisticated approach more widely. Bleached cotton fabrics treated by these two processes were examined for changes in morphology by X-ray analysis, fibre cross-section and measurements of the sorption of iodine, steam and CI Direct Blue 1 (Figure 3.1). Marked differences between the two treatments were found and liquid ammonia swelling does not always result in a stable fibre structure [39]. High-quality cotton shirting subjected to liquid ammonia treatment followed by durable-press finishing showed good easy-care properties with only modest strength loss. Changes in dyeing behaviour and morphology during this sequence were investigated [40] using the direct dyes of markedly different molecular sizes that are recommended for the red–green test to indicate immaturity in raw cotton: CI Direct Red 81 (Figure 3.14) and Green 26 (Figure 3.15). In a further study by the same group of workers, treatment with liquid ammonia followed by caustic soda was compared to treatment with ammonia alone [41]. The changes in fibre structure were examined by X-ray analysis for crystalline fraction and type of lattice, as well as sorption measurements using the disazo dye CI Direct Blue 1. Figure 3.15 NaO3S NaO3S N OH N H3C OCH3 N N OH HN NaO3S SO3Na N NN N H HN N N OH COONa
  • 174. 167 HomeContents 3.3.4 Diagnostic dyeing tests to assess effects of preparation Many latent faults in textile materials only become visible during the dyeing process. The sensitivity of dyes to inherent variations in properties of untreated or processed textiles can be applied in the development of diagnostic dyeing or staining tests to define the nature and extent of such differences [42]. One of the long-established stain tests is based on the violet colour obtained by spotting iodine onto cotton containing residual starch size. Durable violet scales produced with direct and vat dyes or printed pigments can be used to quantify the degree of staining produced by the iodine, which fades rather quickly on storage [43]. Several possible dyeing tests are available to assess the maturity of cotton samples and their response to alkaline pre-treatments. The red–green test at the boil using a mixture of direct dyes of different molecular sizes, disazo CI Direct Red 81 (Figure 3.14) and the tetrasulphonated CI Direct Green 26 (Figure 3.15), has been mentioned already. Another sensitive test of this kind is based on a mixture of the monosulphonated CI Acid Red 151 (Figure 3.16) and the tetrasulphonated CI Direct Blue 10 (Figure 3.17). Untreated cotton is stained red but an increase in concentration of alkali in the causticising or mercerising process results in progressively bluer staining [44]. NaO3S N N N N HO 3.3.5 Dyeing properties of viscose and bast fibres When different cellulosic fibres are dyed from dyebaths containing the same amount of a direct dye, the depth attained on each fibre may differ considerably. The visual differences observed are due partly to the effect of the rate of dyeing, since practical dyeing is seldom carried to equilibrium, and partly to an optical effect arising from different fibre thicknesses. The apparent depth of dyeings Figure 3.16 NaO3S OHOH N SO3Na N H3CO OCH3 N N SO3NaNaO3S OH OH Figure 3.17 DYE STRUCTURE AND DYEABILITY OF CELLULOSIC FIBRES
  • 175. 168 DYEING WITH DIRECT DYES HomeContents produced with the same percentage of dye on filament viscose varies with the linear density of the filament. This is an optical effect, finer filaments appearing lighter in depth due to the greater degree of surface reflection. In order to achieve equal apparent depth on viscose filaments differing only in their linear density, the dye concentration must be inversely proportional to the square root of the linear density. In addition to these effects, however, there remains a substantial difference between the amounts of dye absorbed by the various cellulosic fibres at equilibrium. Since the basic chemical structure is the same for all cellulosic fibres, the energetics of dyeing equilibrium should also be the same. The differences that do exist between the fibre types are in the total micellar surface available for dye adsorption and in the electrical charge on the fibres. In the manufacture of viscose, dissolution of the natural cellulose produces intense swelling and leads to a much greater micellar surface in the finished viscose than in the original cellulose. At the same time oxidative degradation occurs, leading to an increased number of carboxy groups in the viscose. Developments in evaluation of the effects of conventional mercerisation and liquid ammonia treatments on the dyeability of cotton with direct dyes have been paralleled by corresponding studies of the implications of these treatments for other cellulosic fibres. High wet modulus and polynosic modal fibres in 50:50 blends with cotton have been compared for their response to alkaline bleaching, causticisation and mercerisation processes, in terms of the effects on physical properties and dye uptake [45]. Liquid ammonia swelling can be used to enhance the uptake of direct dyes by viscose, either by dyeing from liquid ammonia as the application medium [46– 48] or by pretreatment before conventional aqueous dyeing [49]. The increase in internal surface of viscose film when dyeing from liquid ammonia permits the deposition of dye aggregates of smaller average size than in the case of aqueous dyeing. Liquid ammonia pretreatment of high wet modulus and conventional viscose fibres followed by a water quench produces higher colour yield and uptake of dye than ammonia treatment followed by evaporation in air, although the latter gives better levelling at the dyeing stage. Linen fabrics and blends of cotton with linen and with ramie were scoured, bleached, dyed with direct dyes and resin-finished. Liquid ammonia treatment was given in most cases, in order to determine at which stage this swelling process was most effective. The dyeability and finish performance were invariably enhanced by liquid ammonia pretreatment but the stage that showed optimum improvement was dependent on the fabric type and the improvement sought [50]. The effects of caustic soda mercerisation, either slack or with
  • 176. 169 HomeContents tension, on the direct dye uptake and colour yield of ramie, linen and cotton yarns were investigated. Treated and untreated ramie exhibited lower dyebath exhaustion than cotton or linen, but for a given dye content the visual depth of the dyeing on ramie was greater [51]. 3.3.6 Cationic pretreatment of cellulosic textiles to enhance dyeability During the 1980s there was a notable revival of interest in techniques for deliberately enhancing the dyeability of cellulose with reactive or direct dyes by pretreatment with a greater variety of cationic products, usually based on nitrogen. Three main approaches were adopted: (1) Vinyl grafting. (2) Reaction with cationic reactant molecules. (3) Application of cationic polymers. To date, however, none of these treatments has achieved significant commercial success. Grafting reactions with cellulose by free-radical polymerisation of olefinic monomers have been explored for many years but the development of modified cotton fibres with enhanced dyeability via this route has attracted particular interest recently [52–55]. Thus a redox system consisting of iron(II) ions, thiourea dioxide and hydrogen peroxide was selected for the emulsion polymerisation of combined glycidyl methacrylate/cotton cellulose in the presence of direct dyes containing labile hydrogen atoms. The treated fabric showed appreciable colour depth with fastness comparable to cotton dyed with reactive dyes, together with a considerably improved dimensional stability [52]. Diethylaminoethylated cotton can be prepared by reacting cotton with the tertiary amine 2-chloroethyldiethylamine in the presence of alkali at 95°C. The uptake of direct or reactive dyes by the modified substrate was much higher than on untreated or alkali-treated controls. The diethylaminoethyl substituents acted as a built-in catalyst, capable of initiating fixation of the reactive dye even in the absence of alkali [53]. A more effective approach to the aminisation of cotton fabric involved padding with caustic soda solution, followed by immersion in an acetone solution of epichlorohydrin and triethanolamine. The etherifying agent for cellulose hydroxy groups was assumed to be the reactive tertiary amine formed by the initial condensation between the starting materials [54]. Many of the attempts to fix quaternary nitrogen compounds to cellulose via ether linkages have depended on the use of epoxy derivatives as well as mono- or bis-reactive agents of the haloheterocyclic type [56,57]. The first product of this DYE STRUCTURE AND DYEABILITY OF CELLULOSIC FIBRES
  • 177. 170 DYEING WITH DIRECT DYES HomeContents type on the market was Glytac A (Protex) (Figure 3.18), which reacted with cellulose via the glycidyl group at alkaline pH [58,59]. Cellulose modified in this way can be dyed with acid dyes and shows enhanced uptake of direct or reactive dyes, although some of the early claims of improved wet fastness were later shown to be exaggerated [60,61]. H2C O CH CH2 N CH3 CH3 CH3 Cl– Figure 3.18 More recent developments have been concerned with application of the epichlorohydrin precursor of Glytac A and the evaluation of analogues with larger substituent groups on the quaternary nitrogen, in order to minimise the unpleasant odour of tertiary amine released during application. Later research covered quaternary agents of the haloheterocyclic type, including mono-reactive mono- or bis-quaternary compounds and exploiting either monochlorotriazine or difluoropyrimidine as the reactive group. Such agents react more readily with cellulose and show better thermal stability than the epoxypropyl types. Both classes of monofunctional reactive system, however, share the disadvantages of relatively low substantivity for cellulose; they must be applied by a padding process. More complex multifunctional structures were evaluated by exhaust application and these gave effective enhancement of dye uptake, although there are practical drawbacks to all of these treatments, including hue changes, poor penetration into the fibre [62] and light fastness limitations [63]. Alternatives to the use of quaternary nitrogen compounds in this context have included the application of ethylenediaminetetramethylphosphonic acid to cotton prior to dyeing with a disazo direct dye [64], as well as the reaction of selected arylsulphonium salts with mercerised cotton, in order to confer improved substantivity for direct or reactive dyes [65]. Many cationic polymers have been applied to cellulose with a view to enhancing uptake of direct or reactive dyes and it is considerably more difficult in these instances to interpret the precise mechanism of the interactions involved, apart from the obvious participation of electrostatic forces between the dye anions and the basic groups (often quaternary nitrogen atoms) in the polymer. Quaternary ammonium-substituted polymers can be derived from the reaction of epichlorohydrins with carbamides or aminoalkylimidazoles, from polymethylol- biguanides or quaternised polyurethanes, from the condensation of polyalkylene-
  • 178. 171 HomeContents polyamines with cyanamides or guanidines, or from polyamines such as polyethyleneimine with epichlorohydrin. The application of the polyamide–epichlorohydrin resin Hercosett 125 (Hercules), originally marketed as a shrink-resist treatment for wool, has recently been applied to cotton, mainly with a view to producing a modified fibre suitable for the absorption and fixation of reactive dyes at neutral pH in the absence of salt [66]. Incorporation of thiourea or ethylenediamine into the Hercosett polymer during the application process has beneficial effects on the results obtained. Thiourea addition gives rise to the formation of isothiouronium groups and inhibits crosslinking of the resin, leaving more nucleophilic NH groups as sites for dye reaction [67]. Ethylenediamine promotes crosslinking of the resin but itself provides extra NH groups as dye-reactive sites [68]. These approaches could be equally useful to enhance the uptake of direct dyes. 3.4 APPLICATION PROPERTIES OF DIRECT DYES Direct dyes are usually applied with the addition of electrolyte at or near the boil. Less frequently, dyeing may be carried out at temperatures above the boil, as in package dyeing of blended yarns. An addition of alkali, usually sodium carbonate, may be made with acid-sensitive direct dyes and with hard water. When cellulose is immersed in a solution of a direct dye it absorbs dye from the solution until equilibrium is attained, and at this stage most of the dye is taken up by the fibre. The rate of absorption and equilibrium exhaustion vary from dye to dye. The substantivity of the dye for cellulose is the proportion of the dye absorbed by the fibre compared with that remaining in the dyebath. 3.4.1 Absorption of direct dyes by cellulose The structural features of direct dyes that confer high substantivity for cellulose have been discussed already (section 3.3.1). Measurements of the absorption spectra of azo direct dyes on cellulose film have shown that dyes enter the cellulose as single molecules and then aggregate inside the cellulose. One method of analysing the absorption of direct dyes by cellulose is to use the diffuse absorption model [69]. This method can account adequately for the absorption of mixtures of direct dyes by cellulose, provided that no interaction occurs between the dyes in either phase and that allowance is made for ionised carboxy groups in the cellulose [70]. APPLICATION PROPERTIES OF DIRECT DYES
  • 179. 172 DYEING WITH DIRECT DYES HomeContents 3.4.2 Dyeing kinetics and dye structure The rates of uptake of direct dyes by cellulose vary extremely widely [5]. The time of half-dyeing (the time to reach an exhaustion level half of that attained at equilibrium) on viscose for the disazo J acid derivative CI Direct Red 23 (Figure 3.19) is approximately 400 times that for the rapidly diffusing stilbene dye CI Direct Yellow 12 (Figure 3.5). A decisive factor in determining the rates of dyeing of many direct dyes from aqueous salt solutions is their tendency to aggregate (section 3.2.5), since this greatly retards their mobility into the water-swollen voids of the substrate. 3.4.3 Classification according to dyeing properties The variations in behaviour between individual direct dyes necessitate care in selection for mixture recipes, in order to achieve optimum results and to prevent the occurrence of faults, such as uneven or insufficiently penetrated dyeings on all types of materials and listing or ending with jig-dyed fabrics. Considerable work was carried out by Courtaulds in the 1940s [71,72] and later by ICI [73] and an SDC committee [74] to characterise the dyeing behaviour of individual direct dyes and thereby enable the best selection to be made for a particular dyeing method, highlighting the parameters to be observed in controlling the dyeing cycle. The time of half-dyeing is an indication of the rate at which a direct dye is absorbed by the fibre. Dyes exhibiting a similar time of half-dyeing were thus regarded as the preferred choice in mixtures. It was found later, however, that rate of dyeing alone is insufficient to predict compatibility and that rate of migration and salt controllability are of greater importance [73]. From this work it was concluded that determination of four parameters was necessary, i.e. migration (or levelling power), salt controllability and the influence of temperature and of liquor ratio on exhaustion. Direct dyes were classified accordingly as follows: N N OH NaO3S N H C N H O OH SO3Na N N NHCOCH3 Figure 3.19
  • 180. 173 HomeContents (1) Class A – dyes that are self-levelling, i.e. dyes of good migration or levelling properties. (2) Class B – dyes that are not self-levelling, but which can be controlled by addition of salt to give level results; they are described as salt-controllable. (3) Class C – dyes that are not self-levelling and which are highly sensitive to salt, the exhaustion of these dyes cannot adequately be controlled by addition of salt alone and they require additional control by temperature; they are described as temperature-controllable. Widespread use was made of the SDC classification in batchwise dyeing but it proved of much less value in the selection of compatible dyes for padding and jig dyeing processes. This can be done, however, by carrying out simple dip or strike tests in which fabric or yarn samples are dyed for a few minutes, removed from the dyebaths, replaced by fresh samples and the procedure repeated several times; the specimens are mounted in series and assessed visually for change of hue and depth. 3.4.4 Dyebath variables that influence dyeing behaviour The principal parameters affecting the absorption of direct dyes by cellulose from aqueous solutions are temperature, time of dyeing, liquor ratio, solubility of the individual dye, salt controllability and to a lesser degree the influence of auxiliary agents. Temperature Strike and fibre penetration are governed by application temperature and are improved by an increase in temperature. Dyeing above the boil has the advantage of shortening the period at top temperature and producing more level and better penetrated dyeings, an important factor when dyeing yarn packages. The effect of an increase in temperature is to increase the dyeing rate but to decrease the equilibrium exhaustion. Consequently, for a fixed dyeing time there is an optimum temperature at which absorption is at a maximum attainable level. Time of dyeing The production of level and well-penetrated dyeings is usually favoured by an increased time of dyeing, although prolonged dyeing at the boil when several successive additions of dye are made for matching purposes sometimes results in the decomposition of direct dyes (section 3.2.6). APPLICATION PROPERTIES OF DIRECT DYES
  • 181. 174 DYEING WITH DIRECT DYES HomeContents Liquor ratio Dyebath exhaustion is governed by liquor ratio but other factors such as solubility of dyes in water, levelling properties and strike have to be taken into consideration. Appreciable variations in liquor ratio apply in dyeing cotton, viscose and other cellulosic fibres. Padding processes operate at very low liquor ratios (2:1 or less), jig methods operate at 3:1 to 5:1, and loose stock or yarn on wound packages are dyed at about 10:1. Longer liquor ratios are operative when dyeing fabrics in winches (20:1 to 30:1), overflow machines or jets (5:1 to 15:1). Dye solubility Dyes of good solubility are preferred for package dyeing and particularly at the low temperatures and liquor ratios necessary in padding processes. The water supply for pad liquors should have a low electrolyte content. Salt controllability The extent to which direct dyes are affected by the addition of electrolytes to the dyebath is known as salt sensitivity. Direct dyes vary appreciably as regards the effect of electrolytes. The addition of electrolyte increases the rate of strike of the dye. In general, the lower the ratio of relative molecular mass to number of sulphonate groups per molecule, the less absorption can take place without electrolyte addition. The commonly used electrolytes are Glauber’s salt (sodium sulphate) and common salt (sodium chloride), the latter being preferred with hard water. Glauber’s salt may cause precipitation of calcium sulphate on the dyed material, resulting in a somewhat harsh handle. The role played by various electrolytes in promoting dyebath exhaustion has been examined in relation to dye structural features. Cotton cellulose was dyed under neutral conditions with two purified direct dyes differing in relative molecular mass and degree of sulphonation, in the presence of various electrolytes and a phosphate buffer system. Under these conditions it was observed that the uni-divalent electrolytes, e.g. calcium chloride, were more effective than other types in promoting dyebath exhaustion [75]. The effect of alkali-metal ions, quaternary ammonium ions, bivalent and trivalent cations on the absorption of CI Direct Violet 66 (Figure 3.11) by viscose fibres at various temperatures has been examined [76,77]. Dye absorption does not seem to depend on the charge of the cations present but only on their size. A reasonably good linear relationship was found between the saturation values in the presence of various ions and their ionic radii. The rate of absorption also
  • 182. 175 HomeContents increased with an increase in the size of the cation, especially at low temperatures. The difference in rate and extent of absorption in relation to ion size tended to decrease as the temperature increased. Equilibrium adsorption isotherms and rates of dyeing of CI Direct Blue 1 (Figure 3.1) on viscose fibres at various temperatures were determined in the presence of electrolytes having predictably different capacities to modify water’s structural characteristics. The results clearly demonstrated that variations in water molecular clustering around the hydrophobic portions of the dye molecules and the cellulosic fibre surface influence the dye-binding mechanism [78]. In the presence of single electrolytes the absorption of CI Direct Blue 1 by viscose fibres and the rate of dyeing at constant electrolyte concentration increased in the order: LiCl NaCl KCl. In binary mixtures of these electrolytes the cation with the greater ability to disrupt the clustering of water molecules (the ion with the larger radius) could considerably enhance the ability of the smaller cation to influence dye absorption and dyeing rate [79]. CI Direct Red 28 (Figure 3.8) is a highly aggregated dye. When it is applied to viscose fibres in the presence of electrolytes capable of modifying water’s structural characteristics in a predictable manner, the absorption and rate of dyeing are very close to those for much less aggregated anionic dyes. Thus although this dye shows a strong tendency to form large aggregates by electrostatic attraction between cationic ammonium and anionic sulphonate substituent groups, the influence of electrolyte cations on the clustering of water molecules around dye ions and the substrate surface is unaffected [80]. Influence of auxiliaries on dye sorption The effect of anionic and nonionic surfactants on the state of aggregation of direct dyes in solution has been studied. Nonionic agents tend to inhibit the formation of larger aggregates and increase the proportion of dye in the monomolecular form. Solubilisation of direct dyes by nonionic agents proceeds effectively up to the critical micelle concentration of the surfactant. Thus concentrated solutions of direct dyes can be stabilised using nonionic additives but the absorption of the dyes by cellulose is decreased when nonionic agents are present [81]. 3.5 BATCHWISE APPLICATION OF DIRECT DYES TO CELLULOSIC TEXTILES Direct dyes are dissolved by pasting with cold water, then adding boiling water BATCHWISE APPLICATION OF DIRECT DYES TO CELLULOSIC TEXTILES
  • 183. 176 DYEING WITH DIRECT DYES HomeContents with stirring. To ensure that dissolution is complete, the solution formed should be sieved before addition to the dyebath. A wetting agent (0.5–1 g l–1) should be added to the dyebath to assist penetration and level dyeing. Direct dyes are normally applied to cellulosic fibres at the boil. The dyebath is set at 40°C, raised to the boil at 2 degC min–1 and maintained at the boil for 30–45 min, during which 10–15 g l–1 of sodium chloride or calcined Glauber’s salt should be added. Pastel hues are preferably dyed without addition of salt. An addition of 0.5–2 g l–1 sodium carbonate may be advantageous when applying dyes of only moderate solubility in full depths. Improved yields can be achieved when applying full depths by cooling to 80°C at the end of the period at the boil, adding a further 5 g l–1 salt and rising to the boil again. There is increasing interest in the controlled dosage of salt in solid form in the dyeing of cotton yarns and fabrics. The effects on levelness in package dyeing of the type of salt dosage, process and machine design factors have been investigated. Electrical resistance measurements were used to monitor salt concentrations in the dyebath [82]. Many direct dyes are suitable for application by combined scouring and dyeing of either woven fabrics on jigs or knitted fabrics on jets or winches. In this process the usual practice is to employ soda ash and a nonionic detergent. Dyes containing amide groups should be avoided because of the risk of hue change owing to alkaline hydrolysis. Combined peroxide bleaching and dyeing with selected direct dyes is another long-established process. It offers savings of process time and energy but more care is necessary to ensure satisfactory levelness and reproducibility. The essential factors are thorough and effective preparation, as well as selection of suitable dyes. Sodium carbonate is preferred to caustic soda because the risk of oxidative degradation of the dyes is greater at higher pH. An organic stabiliser for the peroxide is preferred to sodium silicate, which tends to confer harshness of handle on terry towelling, for example. This process is of special interest for dyeing pastel colours and Class A or B dyes are preferred to attain satisfactory levelling. Components of green mixtures should be carefully chosen because of the risk of catalytic fading; certain yellow direct dyes fade more quickly in the presence of phthalocyanine or triphenodioxazine blues. Copper-complex azo direct dyes are sensitive to oxidation and cause catalytic decomposition of hydrogen peroxide. They are unsuitable for inclusion in recipes for the combined bleaching and dyeing process. Copper phthalocyanine blues, on the other hand, do not catalyse peroxide decomposition.
  • 184. 177 HomeContents 3.6 SEMI- AND FULLY CONTINUOUS DYEING PROCESSES FOR DIRECT DYES Direct dyes are much less suitable for continuous application than for batchwise dyeing. Tailing is a serious problem in continuous dyeing and prolonged diffusion is necessary in pad–batch or pad–steam methods. As far as possible, close attention should be given to selecting dyes with similar absorption characteristics and to controlling the rate of supply of feed liquors. The wet fastness of direct dyes may be lower when they are applied by continuous processes than by conventional batchwise methods, but this property can be maximised by adding electrolyte or by increasing the impregnation temperature. The pad–roll process is probably the most suitable semi-continuous method of dyeing cellulosic fabrics with direct dyes at 80–100°C. Problems with listing and ending have been minimised in the Variflex S pad–roll unit with i.r. heating [83]. Advice has been given with regard to control of fabric moisture content and pad liquor temperature, dye selection for optimum compatibility and the use of appropriate auxiliaries [84]. The pad–roll system is more versatile in production conditions than are steaming methods but the development of compact pad–steam ranges in recent years has enabled more valid cost comparisons to be reached against the semi- continuous alternatives [85]. Padding assistants that facilitate the application of direct dyes to cellulosic fabrics without tailing problems have been developed. A method for determining in advance the exchange factor in the pad dyeing of cotton and viscose fabrics with direct dyes has been described [86]. 3.7 AFTERTREATMENT PROCESSES FOR DIRECT DYEINGS The wet fastness properties (particularly washing, water and perspiration) of virtually all dyeings of direct dyes are inadequate for many end uses but notable improvements can be brought about by aftertreatments. All such treatments, however, incur increased processing costs because of the extra time, energy, labour and chemicals involved. 3.7.1 Diazotisation and development Many long-established direct dyes containing primary amino groups could be diazotised and coupled on the fibre with a variety of developers, including naphthols (e.g. 2-naphthol), diamines (e.g. m-phenylenediamine) and phenols, to give larger molecules with improved wet fastness properties. A change in hue often occurred, depending on the developer employed, and frequently the light AFTERTREATMENT PROCESSES FOR DIRECT DYEINGS
  • 185. 178 DYEING WITH DIRECT DYES HomeContents fastness was impaired. Diazotised and developed direct dyes were widely used at one time for the production of dischargeable ground colours on cotton and viscose fabrics. They have now been largely replaced by reactive dyes. The complexity of this approach and the associated problems of hue change and light fading have rendered this technique virtually obsolete. 3.7.2 Metal salt treatments The aftertreatment of dyeings with metal salts to confer improved fastness was practised by dyers long before direct dyes were discovered [1]. Treatment with acidified copper salt solution (typically with 0.25–2% copper sulphate and 1% acetic acid for 20–30 min at 60°C) results in a marked improvement in the light fastness of certain direct dyes, e.g. CI Direct Blue 1 (Figure 3.1), which is converted into the derived copper complex (Figure 3.2). Subsequent washing or alkaline treatment removes the copper, however, so that the light fastness reverts to the original rating. Ranges of direct dyes have been produced that are of no commercial interest in the uncoppered state, but when aftertreated with copper salts give dyeings of reasonable light and wet fastness. Many of the dyes of this type are pH-sensitive before coppering and this necessitates care in handling. Control of application is essential, since once coppering has taken place, unlevel results can only be rectified by stripping. Apart from chelation taking place with certain direct dyes containing two hydroxy groups in positions ortho to the azo group, it is also possible with dyes containing other groups such as o-hydroxy-o-methoxy (Figure 3.1), o-hydroxy- o-carboxy or salicylic acid groups. Instead of copper sulphate, treatment with chromium fluoride or acetate, or sodium or potassium dichromate, will enhance wet fastness, typically using 1–2% of the chromium salt and 1–2% acetic acid for 20–30 min at 60–80°C. Treatment with a mixture of copper sulphate and a dichromate will improve both light and wet fastness of certain direct dyes. 3.7.3 Cationic fixing agents These compounds interact with the sulphonate groups present in direct dyes, conferring increased wet fastness in all tests at temperatures below 60°C. They will also precipitate direct dyes from solution, and therefore the dyed material must be cleared of loosely held dye before treatment. Hue changes may occur and, in some cases, light fastness may be reduced. The dye–auxiliary complex usually dissociates in hot detergent solutions above 60°C. Many such agents are available, including quaternary ammonium or pyridinium compounds, amide-
  • 186. 179 HomeContents formaldehyde adducts, complex fatty amides and melamine derivatives. Many of the cationic polymers evaluated as pretreatments to enhance the dyeability of cellulosic materials (section 3.3.6) are also applicable as conventional aftertreatments to improve wet fastness properties. 3.7.4 Formaldehyde treatment Treatment of certain direct dyeings, mainly blacks, with 2–3% formaldehyde (30%) and 1% acetic acid (30%) for 30 min at 70–80°C improves the wet fastness (to both water and washing) of the dyeing. A drop of light fastness may occur, however. The improvement in wet fastness is most pronounced with dyes containing an end component with two hydroxy or amino groups meta to each other. CI Direct Black 38 and 166 (Figures 3.9 and 3.10) are examples of dyes suitable for this type of aftertreatment. It is believed that when dyes of this type react with formaldehyde, methylene bridges are formed in the 5-positions of the terminal 2,4-diamino- or 2,4-dihydroxy-phenylazo groupings of two dye molecules. These dimeric products have lower solubility in water than the parent dye molecules. 3.7.5 Crosslinking agents and resin treatments Improvements in wet fastness properties can be ensured by treatment with cellulose reactants or amide-formaldehyde resins. Subsequent removal of the resin by acid hyrolysis (e.g. formic acid at 90°C or hydrochloric acid at 60°C) leaves the unfixed direct dye on the fibre with its originally low level of wet fastness. Treatment with crosslinking agents in resin finishing improves the wet fastness but hue and light fastness may be adversely affected. 3.7.6 Comparisons of aftertreatments for direct dyeings In a recent investigation, bleached and mercerised cotton poplin was dyed with the trichromatic series of CI Direct Yellow 12 (Figure 3.5), Red 81 (Figure 3.14) and Blue 1 (Figure 3.1). Four cationic dye-fixing agents, a long-chain polyamine and copper sulphate were applied as aftertreatments. The polyamine gave consistently better wet fastness than dicyandiamide-formaldehyde adducts. Fastness performance varied with the number of sulpho groups and hydrogen- bonding sites and their positions in the dye molecule, as well as the active agent content of the fixing agent [87]. AFTERTREATMENT PROCESSES FOR DIRECT DYEINGS
  • 187. 180 DYEING WITH DIRECT DYES HomeContents Efficiency indices have been devised to assist users of direct dye-fixing agents to evaluate their cost-effectiveness in improving wet fastness, but also taking into account their adverse effects on hue and light fastness [88]. Fastness to wet rubbing can be a serious problem in full depths of direct dyes aftertreated with a cationic agent in the usual way. Mechanical destruction of the dyed fibres produces microscopically small fragments that stain the adjacent white fabric. Aftertreating agents themselves do not significantly modify fastness to wet rubbing, however [89]. 3.7.7 Application of reactant-fixable direct dyes to cellulose The patent literature relating to Indosol CR (S), the first of the special aftertreating agents developed for the fixation of Indosol SF (S) dyes (see section 3.2.3), revealed that it was a mixture of three components and that it was designed for the continuous process of simultaneous crosslinking and dye fixation. The components were: (1) a polybasic condensate of diethylenetriamine and an amide, such as cyanamide, dicyandiamide, guanidine or biguanide; (2) an N-methylol precondensate of the dicyandiamide-formaldehyde type; (3) a latent acid catalyst, such as lactic acid. The polybasic condensate (1) was later marketed alone as Indosol E-50 (S) for the exhaust aftertreatment of Indosol dyeings; this component did not confer any improvement in crease recovery. Compared with reactive dyes, Indosols showed economic advantages in respect of consumption of dyes, chemicals and water, and of reduced process time. Treatment with Indosol CR eliminated the need for any subsequent resin finishing process. Trends in washing and laundering practice were shown to be favourable towards the introduction of the Indosol system [2–4]. The cold pad–batch process was proved to be eminently suitable for the Indosol SF dyes, using urea and Vicontin I (S) to assist solubility and diffusion during the 12 h batching time [90,91]. A third type of Indosol aftertreating agent was later introduced to achieve even better fastness to washing. Indosol E-F forms the same type of dye–agent complex as is formed from Indosol E-50, but this complex is held in the fibre more securely by virtue of a covalent bond between agent and fibre that resembles the dye–fibre bonds of conventional reactive dyeings. Although Indosol E-F is designed for exhaust application, a final fixation in alkaline solution at 40°C is necessary [92,93].
  • 188. 181 HomeContents 3.7.8 Effluent treatment after direct dyeing Environmental considerations now impinge on every aspect of the dyer’s activities. Although direct dyes are particularly easy to apply, the presence of chelated copper in many structures of above average fastness to light and the frequent need to apply cationic aftertreating agents means that care must be taken to minimise difficulties in effluent treatment. Reviews of water supply and recovery that are of interest to the dyer of cellulosic textiles [94,95] provide information on the sources and conservation of water and the effects of common contaminants on textile processing. Trace metals can affect the performance of many dyes, not least the copper-complex direct types. Although sequestering agents are used to minimise the influence of alkaline earth or transition metal ions on unmetallised dyes, they can obviously exert undesirable effects on the colour and fastness of metal-complex dyes. The rising costs of water procurement and effluent disposal are forcing dyers and finishers to re-examine the potential for repeated use of dyebaths and other process liquors. Equations have been derived to show how electrolytes build up during repeated use of direct dyebaths. They can be used to calculate the amounts to be added after each dyeing or the amount of the bath to be discharged [96]. Studies of the sorption of anionic dyes by activated carbon from dyehouses wastes demonstrated that the saturation adsorption of direct dyes increased with: (1) increasing relative molecular mass of the dye; (2) sodium sulphate concentration in the waste water; (3) integral pore volume of the carbon. Methods of measuring the colour of waste dye liquors and minimising the contribution of dyeing and printing processes to this problem have been reviewed. A survey found that reactive red dyes and sulphur blacks caused most difficulty in this regard, but direct dyes can be readily removed by adsorption or precipitation [97]. Ozone treatment has potential as a means of decolorising exhaust dyebaths for reuse. A dose of 200 mg l–1 ozone was found to remove more than 95% of the colour present [98]. Large-scale trials on dyehouse effluents containing direct and other dyes were successful and a relationship was established between colour removal, pH and ozone consumption [99]. Fundamental studies of the ozonolysis of model azo dyes have deomonstrated that CI Direct Yellow 12 (Figure 3.5) yields mainly 4-carboxy-4′-ethoxyazobenzene-3-sulphonate and hydrogen peroxide [100]. AFTERTREATMENT PROCESSES FOR DIRECT DYEINGS
  • 189. 182 DYEING WITH DIRECT DYES HomeContents 3.8 SIGNIFICANCE OF FINISHING FOR DIRECT DYES ON CELLULOSIC FABRICS Any effect of durable finishes on the hue and fastness properties of direct dyeings depends on the chemicals and treatment conditions employed as well as on the individual dyes. No significant effect on either hue or light fastness of direct dyes has been reported with either fluorocarbon stain-repellent finishes or the acrylate resins often used in soil-resistant finishing. The majority of softening agents applied to cellulosic fabrics, with the exception of those derived from N-methylol compounds, are likely to have any adverse effects on direct dyeings [101]. Although some softeners are mildly cationic, no effect on the light fastness of direct dyes has been reported. Amongst compounds used for water-repellent finishing of cellulosic materials, only those containing melamine derivatives adversely affect either the hue or the fastness of direct dyeings. 3.8.1 Effect of resin precondensates and cellulose reactants on direct dyeings The application of resin finishes is of great importance for conferring dimensional stability to fabrics made from cotton or viscose, either alone or in blends with synthetic polymer fibres, especially polyester. An important benefit of resin finishing is to bring about a marked improvement in wet fastness of direct dyes. In the case of wash fastness, resin-treated direct dyeings will withstand washing at temperatures below 60°C, even in full-depth dyeings. This notable improvement in wet fastness on resin finishing means that direct dyes can be used for many end uses in which they would be quite unsuitable without this treatment. On the other hand, hue may be affected and light fastness may decrease, the effect in both cases varying with the individual direct dye and the crosslinking system. Numerous products and processes are used for crosslinking cellulose; these include cellulose reactants such as dimethylolethyleneurea, dimethyloldihydroxy- ethyleneurea and dimethylolpropyleneurea, as well as urea- and melamine- formaldehyde precondensates. Certain inorganic salts, such as magnesium chloride or zinc nitrate, are used as catalysts in resin finishing, with curing conditions showing wide variations in respect of humidity and temperature. Zinc nitrate has a greater effect on both hue and light fastness than magnesium chloride, but metal-salt catalysts show less effect on light fastness than ammonium salts. Studies of the relationship between dye structure and light fastness after resin treatment have shown that hydroxy and amino groups are normally the most
  • 190. 183 HomeContents sensitive, although their positions in the dye molecule play a significant role. Unbound formaldehyde together with the catalyst reduce the light fastness of naphthylazo dyes with an amino group located ortho to the azo group. The higher the residual amount of unbound formaldehyde on the finished fabric, the greater the adverse effect on light fastness. 3.8.2 Simultaneous dyeing and finishing of cellulosic fabrics An obvious way to minimise processing costs in continuous dyeing and finishing is to try to carry out the two stages simultaneously rather than in sequence. Unfortunately, the moist or wet alkaline fixation conditions necessary for most of the dyeing classes used on cellulosic fabrics (reactive, sulphur, vat, azoic coupling) are highly incompatible with the dry cure in the presence of a latent acid catalyst that is required for all the important cellulose reactant finishes. Direct dyes, on the other hand, do not need alkali addition and they benefit greatly from the improved wet fastness conferred by a crosslinked finish. Cotton fabrics padded with selected acid, direct or disperse dyes, N-methylol reactant and ammonium chloride as catalyst were dried and then cured at 160°C. In general, increasing the number of nucleophilic groups in the dye molecule gave a higher colour yield within a dye class, but molecular size was also significant. Fastness to rubbing and perspiration was good, but light fastness varied from 1 to 6 depending on dye structure and resin type [102]. In a similar investigation, typical direct dyes were applied to cotton together with dimethyloldihydroxyethyleneurea (DMDHEU) reactant and a catalyst. Colour yield increased with curing temperature and the deepest dyeings were achieved using ammonium persulphate as catalyst [103]. The phthalocyanine turquoise CI Direct Blue 86 was padded with DMDHEU and magnesium chloride on untreated, alkali-treated and diethylaminoethylated cotton fabrics. The substrate containing cationic dyeing sites invariably showed the highest colour yield and crease recovery, irrespective of the finish concentration and curing conditions [104]. Various direct and other dyes were applied to cotton in conjunction with various N-methylolacrylamide derivatives and catalysts. The modes of reaction between the dye, reactant and cellulose were interpreted from the colour yields and physical properties of the treated fabrics [105]. 3.8.3 Subsequent dyeing of crosslinked cotton The reorganisation by retailers of fashion garments in recent years to bring their SIGNIFICANCE OF FINISHING FOR DIRECT DYES ON CELLULOSIC FABRICS
  • 191. 184 DYEING WITH DIRECT DYES HomeContents garment orders closer in line with customer demands in terms of colour and style has resulted in a revival of activity in dyeing made-up garments or garment blanks, rather than fabric dyeing. If a resin finish is required to provide easy-care performance, however, this is only feasible as a continuous treatment of the fabric before garment manufacture. Accordingly, there has been a great deal of interest and development, mainly in the USA, of methods of garment dyeing suitable for cotton that has already been crosslinked and is therefore difficult to dye using conventional processes. If dyes are to be applied after finishing, this also puts greater demands than usual on the uniformity of the finish. Cotton fabrics finished with DMDHEU or a methylolated melamine precondensate and various catalysts were then dyed with CI Direct Red 81 (Figure 3.14). In general, the colour yield, bound nitrogen and physical performance of the fabrics cured with semicarbazide hydrochloride were equal or superior to those where conventional inorganic salts were used [106]. Untreated, alkali-treated and diethylaminoethylated cotton poplin fabrics were treated with cold solutions of mineral acids and neutralised before crosslinking with DMDHEU and magnesium chloride and dyeing with a copper-complex disazo direct dye of the dinaphthylurea type. Colour yield, fastness and finish effects were dependent on substrate, type and concentration of mineral acid, and curing conditions for the finish [107]. An alternative approach to applying the crosslinking finish to cellulose that already contains cationic sites is to incorporate the cationic modifier into the resin finish formulation [108]. Thus a formulation containing trimethylol- acetylenediurein (TACD) (Figure 3.20), choline chloride (2-hydroxyethyl- trimethylammonium chloride) and a catalyst was applied to mercerised cotton and cured in the usual way. Tests with CI Direct Blue 1 (Figure 3.1) and CI Reactive Red 2 demonstrated that this route provided an easy-care finish that could be readily dyed in full depths after crosslinking [108]. A subsequent investigation compared choline chloride with methylpolyoxyethylene cocoammonium chloride and with a water-soluble polymer containing grafted quaternary groups and active hydroxy groups. Dyeing with a selected series of dyes, including the components of the red–green test (Figures 3.14 and 3.15), showed that choline-based treatments were superior to the others in terms of colour yield and light fastness retention. Direct dyes invariably gave higher light fastness on the cationic-modified finishes. Factors influencing the dyeability of crosslinked cotton with direct dyes have been studied in more detail [109,110]. Cotton fabrics crosslinked with formaldehyde, dimethylolethyleneurea (DMEU) and DMDHEU were dyed with CI Direct Red 81 (Figure 3.14) and a tetrasulphonated trisazo direct blue. Dye
  • 192. 185 HomeContents uptake was dependent on whether the cotton had been mercerised, the type of crosslinking agent and the dye structure, as well as the curing and dyeing conditions. In general, sorption of the larger trisazo blue dye was favoured by formaldehyde-linked cellulose, whereas sorption by the fibres crosslinked with DMEU or DMDHEU was greater for the smaller disazo red [109]. The pore structures of cotton crosslinked with DMDHEU or 4,5-dihydroxy-1,3- dimethylimidazolidone (DHDMI) (Figure 3.21) were compared using reverse gel permeation chromatography. The results were interpreted in relation to the uptake by these fabrics of CI Direct Red 81 (Figure 3.14). Although crosslinking reduced the accessible internal volume, samples reacted with DHDMI retained substantially more accessible internal volume across the entire range of pore sizes [110]. N N H3C CH3 OHHO O Figure 3.21 N N N N O O HOCH2 HOCH2 CH2OH H Figure 3.20 SIGNIFICANCE OF FINISHING FOR DIRECT DYES ON CELLULOSIC FABRICS Several methods for the production of readily dyeable garments made from easy-care cotton fabrics have been summarised [111–113]: (1) The use of DHDMI to give a finished fabric free from formaldehyde that is dyeable with direct, vat or reactive dyes. (2) Wet treatment with formaldehyde to give Form W cotton, which is more dyeable with direct dyes than untreated cotton but shows poor dry crease recovery. (3) Additives such as choline chloride or triethanolamine to TACD finishes to confer affinity for acid, direct or reactive dyes. (4) Aftertreatment of a urea–formaldehyde finish with 5% acetic acid solution at 60°C to achieve partial hydrolysis and improved dyeability but some sacrifice of finish performance. Cotton crosslinked with DHDMI was dyed with several direct and reactive dyes. Dyeability was adequate with direct dyes of relatively small molecular size but the substantivity of more complex structures for the modified cellulose was much lower.
  • 193. 186 DYEING WITH DIRECT DYES HomeContents Mono-, di- and tri-ethanolamines were compared as additives to a DMDHEU finish on cotton. Triethanolamine was the most effective in enhancing dyeability, giving results approximately equivalent to direct dyes on the untreated fibre. Two carbamoylethylamine adducts, prepared by the reaction of acrylamide with diethylamine or diethanolamine, were also investigated with DMDHEU in a similar way. Nitrogen analyses confirmed that the additives became part of the finish, and if methylolated to render then reactive with cellulose they could be used alone to enhance dyeability with anionic dyes, although they were not capable of crosslinking independently [111–113]. The dyeing characteristics of cotton crosslinked with DMDHEU in the presence and absence of triethanolamine were compared with those of mercerised and untreated controls. Crosslinking reduced most of the kinetic constants (dyeing rate, structural diffusion resistance, maximum exhaustion, activation energy, diffusion coefficient) but the incorporation of triethanolamine reversed these trends and thus enhanced dyeability. It was shown that in the crosslinked cellulose, triethanolamine reacted preferentially with DMDHEU crosslinks rather than with the remaining free hydroxy groups of the substrate [114]. 3.9 REFERENCES 1. C C Cook, Rev. Prog. Coloration, 12 (1982) 73. 2. W B Egger and T Robinson, Chemiefasern und Textilind., 31/83 (1981) 852. 3. W B Egger and T Robinson, Textilveredlung, 18 (1983) 41. 4. W B Egger and T Robinson, Text. Chem. Colorist, 15 (Oct 1983) 189. 5. J Shore, Ed., Colorants and auxiliaries, Vol.1 (Bradford: SDC, 1990). 6. F Jones, D Patterson and D Srinivasan, J.S.D.C., 96 (1980) 628. 7. C V Stead, Rev. Prog. Coloration, 6 (1975) 1. 8. R Krishnan, N Sekar and S Seshadri, Dyes and Pigments, 7 (1986) 457. 9. F Calogero et al., Dyes and Pigments, 8 (1987) 431. 10. H S Freeman, J F Esancy and L D Claxton, Dyes and Pigments, 13 (1990) 55. 11. W B Egger, B Kissling and T Robinson, Melliand Textilber., 62 (1981) 947. 12. W B Egger, B Kissling and T Robinson, Am. Dyestuff Rep., 71 (Sep 1982) 55. 13. M Tiefenbacher, Textil Praxis, 38 (1983) 965. 14. W B Egger, W Joos and C Oschatz, Textilveredlung, 19 (1984) 324. 15. W B Egger, W Joos and C Oschatz, Dyer, 171 (Feb 1986) 16. 16. E Coates, J.S.D.C., 85 (1969) 355. 17. T Vickerstaff, The physical chemistry of dyeing, 2nd Edn (London: Oliver and Boyd, 1954). 18. C C Wilcock, J.S.D.C., 63 (1947) 136. 19. S M Neale and W A Stringfellow, J.S.D.C., 59 (1943) 241. 20. A Schmitz, J.S.D.C., 71 (1955) 910. 21. F H Holmes and H A Standing, Trans. Faraday Soc., 41 (1945) 542. 22. S Fisichella, G Scarlata and M Torre, J.S.D.C., 94 (1978) 521.
  • 194. 187 HomeContents 23. K H Meyer, Melliand Textilber., 9 (1928) 573. 24. H H Hodgson, J.S.D.C., 49 (1933) 213. 25. M D Watson and B W Jones, Text. Chem. Colorist, 17 (Mar 1985) 37. 26. L Cheek, A Wilcock and L H Hsu, Text. Res. J., 56 (1986) 569; 57 (1987) 690. 27. L Cheek and A Wilcock, J.S.D.C., 104 (1988) 477. 28. A Wilcock and D Lloyd-Graham, J.S.D.C., 106 (1990) 104 29. F Hoffmann, Chemiefasern und Textilind, 31/83 (1981) 762. 30. P F Greenwood, J.S.D.C., 103 (1987) 342. 31. A Hebeish, E Allam and A T El-Aref, Textilveredlung, 16 (1981) 174. 32. H Mattyus, I Rusznak and M Ivanyi, Textilveredlung, 17 (1982) 161. 33. K Bredereck and J Wolf, Melliand Textilber., 64 (1983) 902. 34. A A Saafan, Melliand Textilber., 65 (1984) 534; 68 (1987) 678. 35. A A Saafan and A M Habib, Melliand Textilber., 68 (1987) 845. 36. A A Saafan and S T Abdel-Halim, Melliand Textilber., 69 (1988) 511. 37. N Ahmed and K D Tahir, Textile Horizons, 5 (Feb 1985) 20. 38. D Bechter, Textil Praxis, 36 (1981) 417. 39. K Bredereck and A A Saafan, Melliand Textilber., 63 (1982) 510. 40. K Bredereck and G Buschle-Diller, Melliand Textilber., 70 (1989) 116. 41. A A Saafan and M Sakran, Melliand Textilber., 69 (1988) 201. 42. J Park and J Shore, Rev. Prog. Coloration, 12 (1982) 43. 43. U Deschler and G Schmidt, Textil Praxis, 36 (1981) 1331. 44. S Dugal, U Denter and E Schollmeyer, Melliand Textilber., 64 (1983) 494. 45. A Kossina, Melliand Textilber., 64 (1983) 746. 46. Y U Kalinnikov, A N Rozhentsev and B N Melnikov, Nov. tekhn. i tekhnol. otdeloch, pr-va. Ivanovo (1984) 33. 47. S M Prusova and Y U Kalinnikov, Tekhnol. tekstil. prom., 159(3) (1984) 60. 48. S M Prusova, Y U Kalinnikov and F Y Telegin, Tekhnol. tekstil. prom., 167(5) (1985) 50. 49. L Cheek and C M Ladisch, Cell. Chem. Technol., 18 (1984) 313. 50. G Kratz and A Funder, Melliand Textilber., 68 (1987) 775. 51. L Cheek and I Roussel, Text. Res. J., 59 (1989) 541. 52. A Hebeish, A Waly and A M Hassanien, Am. Dyestuff Rep., 75 (Apr 1986) 26. 53. E A El Alfy et al., Am. Dyestuff Rep., 75 (May 1986) 22. 54. A Waly et al., Am. Dyestuff Rep., 79 (July 1990) 34. 55. A Hebeish and A Higazy, Am. Dyestuff Rep., 77 (Feb 1988) 34. 56. R L Stone and R J Harper, AATCC International Conference and Exhibition (1986) 214. 57. T Robinson, Melliand Textilber., 68 (1987) 137. 58. M Rupin, G Veaute and J Balland, Textilveredlung, 55 (1970) 829. 59. M Rupin, Text. Chem. Colorist, 8 (1976) 139. 60. N J J van Rensburg, SAWTRI Bull., 13 (June 1979) 14. 61. D F Herbert, F A Barkhuysen and N J J van Rensburg, SAWTRI Report No. 524 (Aug 1983). 62. G E Evans, J Shore and C V Stead, J.S.D.C., 100 (1984) 304. 63. D M Lewis and X P Lei, Text. Chem. Colorist, 21 (Oct 1989) 23. 64. Y Riad, H M El-Nahas and H M Hamza, J.S.D.C., 107 (1991) 144. 65. T L Vigo and E J Blanchard, Text. Chem. Colorist, 19 (June 1987) 19. 66. S M Burkinshaw X P Lei and D M Lewis, J.S.D.C., 105 (1989) 391. 67. S M Burkinshaw et al., J.S.D.C., 106 (1990) 307. 68. X P Lei and D M Lewis, J.S.D.C., 106 (1990) 352. 69. R McGregor, Text. Res. J., 42 (1972) 536. REFERENCES
  • 195. 188 DYEING WITH DIRECT DYES HomeContents 70. A Johnson, N M Patel and R H Peters, J.S.D.C., 90 (1974) 50. 71. J Boulton and B Reading, J.S.D.C., 50 (1934) 381. 72. J Boulton J.S.D.C., 60 (1944) 5. 73. D R Lemin, E J Vickers and T Vickerstaff, J.S.D.C., 62 (1946) 132. 74. SDC Committee on Direct Dyes, J.S.D.C., 62 (1946) 280; 64 (1948) 145. 75. A O Noah, C M O A Martius and J A Braimah, J Appl. Polym. Sci., 32 (1986) 5841. 76. M R De Giorgi, G Alberti ang G Seu, Ann. Chim. (Rome), 73 (1983) 635. 77. M R De Giorgi, G Alberti and A Cerniani, Am. Dyestuff Rep., 74 (Mar 1983) 33. 78. S R Sivaraja Iyer and K Subramanian, J.S.D.C., 96 (1980) 185. 79. S R Sivaraja Iyer and G Ramaseshan, J.S.D.C., 103 (1987) 170. 80. S R Sivaraja Iyer and R Raghunath, Indian J. Text. Res., 13 (Mar 1988) 49. 81. L P Loseva et al., Tekhnol. tekstil prom., 5 (1987) 79. 82. A Lubbers, I Souren and H K Rouette, Melliand Textilber., 71 (1990) 371. 83. Sacconaghi Monaco SA, Tinctoria, 85 (Nov 1988) 98. 84. H Lehmann, Melliand Textilber., 67 (1986) 189. 85. F Somm and R Buser, Textilveredlung, 17 (1982) 15. 86. S Dugal, K F Elgert and G Heidemann, Melliand Textilber., 68 (1987) 61. 87. N Bhattacharyya et al., Am. Dyestuff Rep., 79 (Mar 1990) 24. 88. J N Etters, Am. Dyestuff Rep., 78 (Sept 1989) 19. 89. H Herlinger and G Schultz, Melliand Textilber., 71 (1990) 126. 90. W B Egger, W Joos and C Oschatz, Textilveredlung, 19 (1984) 324. 91. W B Egger, W Joos and C Oschatz, Dyer, 171 (Feb 1986) 16. 92. J Varghese et al., BTRA Scan, 18 (Sept 1987) 5. 93. J A Hook and A C Welham, J.S.D.C., 104 (1988) 329. 94. B Smith and J Rucker, Am. Dyestuff Rep., 76 (July 1987) 15; 76 (Aug 1987) 68. 95. J Park and J Shore, J.S.D.C., 100 (1984) 383. 96. B G Nazarov, E P Fazullina and T G Favorskaya, Tekstil. prom., 6 (June 1986) 47. 97. B W L Textil, Textilveredlung, 20 (1985) 123. 98. S V Zima, A I Pivtorak and L I Tikachenko, Tekstil. prom., 6 (June 1986) 49. 99. J M Green and C Sokol, Am. Dyestuff Rep., 74 (Apr 1985) 50. 100. G Seu and G Alberti, Ann. Chim. (Rome), 74 (1984) 27. 101. G L Alekseeva and M A Bychhova, Tekstil. prom., 50 (Jan 1990) 49. 102. M M Kamel, E A Kharadly and B M Youssef, Am. Dyestuff Rep., 72 (Dec 1983) 40. 103. N A Ibrahim and K Haggag, Am. Dyestuff Rep., 75 (Apr 1986) 20. 104. A Hebeish, E A El Alfy and M H A Mardini, Am. Dyestuff Rep., 76 (Oct 1987) 42. 105. M M Kamel et al., Am. Dyestuff Rep., 79 (Mar 1990) 50, 62. 106. N A Ibrahim et al., Am. Dyestuff Rep., 75 (May 1986) 13. 107. A Hebeish, E A El Alfy and M H A Mardini, Am. Dyestuff Rep., 76 (July 1987) 33. 108. R J Harper and R L Stone, Text. Chem. Colorist, 18 (Nov 1986) 33. 109. E J Gonzales and E J Blanchard, Am. Dyestuff Rep., 77 (Mar 1988) 15. 110. N R Bertoniere and W E King, Text. Res. J., 59 (1989) 608. 111. E J Blanchard and R M Reinhardt, Am. Dyestuff Rep., 77 (Jan 1988) 29; 78 (Feb 1989) 30; 79 (June 1990) 15; 79 (July 1990) 27. 112. E J Blanchard and R M Reinhardt, Ind. Eng. Chem. Red., 28 (1989) 490. 113. E J Blanchard and R M Reinhardt, Text Chem. Colorist, 21 (Mar 1989) 19. 114. J Chrastil, R M Reinhardt and E J Blanchard, Text. Res. J., 60 (1990) 441. REFERENCES
  • 196. 189 HomeContents 189 CHAPTER 4 Dyeing with reactive dyes John Shore 4.1 INTRODUCTION The discovery in 1884 of substantive dyes that could be applied directly to cotton without prior treatment with a mordant greatly simplified subsequent developments in the dyeing of cellulosic fibres. It simultaneously initiated a prolonged search for novel approaches and techniques that would satisfy consumer demand for dyed materials that withstand the severe washing conditions to which cotton textiles are normally subjected. The vat, sulphur and azoic ranges of dyes, all developed mainly in the three decades following the discovery of the first direct dye, depended on various techniques for depositing insoluble colorants inside the amorphous regions of the cellulosic polymer. For a further four decades, this remained the only feasible method of achieving dyeings of high fastness to washing on cellulosic textiles. During this period much fundamental work on the structure of cellulose and the morphology of cellulosic fibres was undertaken and numerous ethers and esters of cellulose were prepared. It would be surprising if none of these had involved the formation or attachment of coloured sidechain substituents on the cellulose chain. Although such necessarily complex and esoteric reactions did indeed confirm the formation of covalent bonds between typical chromophoric groups and the hydroxy groups in the cellulose molecule, they remained essentially of academic interest only [1,2]. Surprisingly, few attempts were made to adapt such reactions or to develop appropriate reagents that would allow such derivatives to be formed under typical dyehouse conditions. Some of the drawbacks of the treatments applied in this period, apart from their multi-stage complexity or the use of costly and hazardous solvent media, were degradative attack of the cellulose chains by some of the vigorous reagents or reaction conditions necessary, or sensitivity of the colorant–fibre bond to hydrolytic attack during subsequent handling or storage of the coloured product [1,3].
  • 197. 190 DYEING WITH REACTIVE DYES HomeContents 4.2 HISTORICAL BACKGROUND It was not until the early 1950s, following the marketing in 1952 by Hoechst of two Remalan (HOE) vinylsulphone dyes capable of reacting with wool, that ICI was successful in devising a reactive dyeing process that enabled cellulose to be dyed with a trichromatic mixture of dyes under practical conditions. Cotton fabric was pretreated with alkali and dried before immersion in a solution of the highly reactive dichlorotriazine dyes. Various refinements of the process were necessary (adding salt to enhance substantivity, lowering the pH and buffering the dyebath to minimise dye hydrolysis) before these novel Procion (ICI, now Zeneca) dyes could be marketed in 1956. Exploitation of the dichlorotriazine reactive system soon led to parallel development of the much less reactive monochlorotriazine dyes, readily made by a substitution reaction between an arylamine and the dichlorotriazine precursor (Scheme 4.1). More stable padding liquors could be prepared using the aminochlorotriazine types and the range of reactivities offered by these two classes of dyes in combination with various alkalis greatly extended the scope of novel continuous dyeing methods for them. At this stage, however, the limitations of continuous dyeing requirements became a temporary constraint on the adoption of reactive dyeing and attention turned to the development of batchwise methods. It was quickly demonstrated Scheme 4.1 N N HNOH NaO3S SO3Na SO3Na N N N Cl Cl N N HNOH NaO3S SO3Na SO3Na N N N HN Cl aniline CI Reactive Red 1 (dichlorotriazine) CI Reactive Red 3 (aminochlorotriazine)
  • 198. 191 HomeContents that optimal temperatures of dyeing should be sought (40°C or lower for dichlorotriazines, 70°C or higher for aminochlorotriazine dyes). The major breakthrough came when it was realised that a neutral exhaustion in salt solution to achieve moderate uptake should precede the alkali addition to promote further exhaustion at a controlled rate determined by the dye–fibre reaction that proceeds at an optimal alkaline pH and temperature. Over the decades since the commercial introduction of reactive dyes, their use has grown steadily rather than spectacularly [4]. When they first appeared it was predicted that reactive dyes would largely replace azoic combinations, direct dyes and sulphur dyes, displace vat dyes from outlets where fastness to bleaching was not essential and eventually dominate the dyeing of cellulosic materials. This did not occur and even in the relatively sophisticated markets they do not account for more than 30% of all dyes consumed on cellulose. World-wide, the traditional uses of direct and sulphur dyes on woven cotton fabrics remain dominant and reactive dyes only account for about 10% of total consumption on this basis of comparison [1,4]. In the USA, however, where vat dyes have been used preferentially for fast-dyed cottons, there has been a more gradual trend in favour of reactive dyes. Greater demand for brilliant hues, a shift towards cotton and cotton-rich blends for apparel and a greater prevalence of short-run lots in the US textile industry have all contributed to this trend. Although the dyeing cycles of direct and reactive dyes are broadly similar, a major difference becomes apparent when unfixed or hydrolysed reactive dye has to be washed off thoroughly in order to achieve the desired superior wet fastness of the reactive dyeing. As much as 50% of the total cost of a reactive dyeing process must be attributed to the washing-off stages and treatment of the resulting effluent. This aspect of the process should be recognised as a major limitation that prevents reactive dyes from achieving the degree of success that was predicted for them at the time of their discovery [1]. Certain other deficiencies are associated with the limited stability of specific types of dye–fibre bond to various conditions of treatment of the dyed fibres (section 4.8). Research into novel reactive dyeing systems and application methods has remained highly active and some notable developments have taken place during recent years. Aminofluorotriazine dyes have joined the vigorous competition between the major reactive systems (section 4.3), and bifunctional systems reacting by two distinctly different mechanisms (section 4.3.2) have also appeared. Bis(aminonicotinotriazine) dyes that react with cellulose under neutral conditions and reactive dyes containing phosphonic acid groups capable of fixation under hot, dry, acidic conditions represent two quite different novel approaches to reactive dye fixation that have been commercialised. HISTORICAL BACKGROUND
  • 199. 192 DYEING WITH REACTIVE DYES HomeContents Pretreatment of cellulosic materials with cationic agents of various kinds that enhance uptake of anionic dyes and facilitate the fixation of reactive dyes in the absence of either salt or alkali [5,6] has also attracted considerable interest (section 3.3). Novel chromophores, cold-dissolving granules and liquid brands (section 4.4), improved automation of application methods (sections 4.5 and 4.6) and greater awareness of the environmental impact of reactive dye wastes (section 4.9) have all received attention over this recent period. 4.3 REACTIVE SYSTEMS The four characteristic features of a typical reactive dye molecule are: (1) the chromophoric grouping, contributing the colour and much of the substantivity for cellulose; (2) the reactive system, enabling the dye to react with the hydroxy groups in cellulose; (3) a bridging group that links the reactive system to the chromophore; (4) one or more solubilising groups, usually sulphonic acid substituents attached to the chromophoric grouping. In a few cases the reactive grouping is attached directly to the chromophore and most reactive systems contain a heterocyclic ring that contributes some substantivity for cellulose. The nature of the bridging group and other substituents on the heterocyclic ring greatly influences the reactivity and other dyeing characteristics of such dyes [7]. The sulphatoethylsulphone precursor of the vinylsulphone reactive group contributes significantly to the aqueous solubility of reactive dyes of this type. Many reagents can be used to acylate cellulose when it is partially ionised under alkaline conditions but in the production of reactive dyes of commercial interest numerous factors other than the chemistry of such reactions have to be taken into account [8]. Some of the most important are the following: (1) Economy – any reactive system selected as the basis of a range of dyes must enable them to be produced at acceptable cost. (2) Availability – the system selected must be free from patent restrictions, health hazards or other limitations to exploitation. (3) Facility – it must be possible to attach the reactive system to the dye chromophoric groupings readily in manufacture. (4) Storage stability – the dye containing the reactive groups must be stable to storage under ambient conditions.
  • 200. 193 HomeContents (5) Efficiency – the yield in manufacture of the reactive dye must be efficient and the dye fixation must be high under conventional conditions of application. (6) Bond stability – the dye–fibre bond must be reasonable stable under severe conditions of washing and durable finishing. Only a few reactive systems (Table 4.1) have met these requirements sufficiently well to become commercially established in a significant segment of the market for reactive dyes on cellulosic fibres. In addition to these important types, several others have been marketed [2,3,9] as alternative ranges that have failed to maintain a foothold in the market-place, or as individual members of established ranges where they show reactivity characteristics similar to one of the more important reactive systems. These systems of relatively minor significance include: 2-alkoxy- 4-chloro-s-triazine, 2,4-dichloropyrimidine-5-carbonylamino, 5-cyano-2,4-di- chloropyrimidine, 5-chloro-2-fluoro-4-methylpyrimidine, 5-chloro-4-methyl-2- methylsulphonylpyrimidine, 1,4-dichloropyridazine-5-carbonylamino, 2-chloro- benzothiazole-6-sulphonylamino, sulphatoethylsulphamoyl and sulphato- propionylamino. During the early years of development of reactive dyes it was soon recognised that the important reactive systems could be classified into two distinct categories, depending on the mechanism of formation of the dye–fibre bond and the stability of this bond to subsequent treatments [8,10]. Those based on nitrogen-containing Table 4.1 Important reactive systems ________________________________________________________________ System Typical brand name ________________________________________________________________ Monofunctional Dichlorotriazine Procion MX (Zeneca) Aminochlorotriazine Procion H (Zeneca) Aminofluorotriazine Cibacron F (CGY) Trichloropyrimidine Drimarene X (S) Chlorodifluoropyrimidine Drimarene K (S) Dichloroquinoxaline Levafix E (BAY) Sulphatoethylsulphone Remazol (HOE) Sulphatoethylsulphonamide Remazol D (HOE) Bifunctional Bis(aminochlorotriazine) Procion H-E (Zeneca) Bis(aminonicotinotriazine) Kayacelon React (KYK) Aminochlorotriazine-sulphatoethylsulphone Sumifix Supra (NSK) Aminofluorotriazine-sulphatoethylsulphone Cibacron C (CGY) ________________________________________________________________ REACTIVE SYSTEMS
  • 201. 194 DYEING WITH REACTIVE DYES HomeContents heterocyclic rings bearing halogeno substituents undergo nucleophilic substitution (Scheme 4.2). The heteroatoms in the aryl ring activate the system for nucleophilic attack because of their electronegativity. The attacking nucleophile can be either a cellulosate anion or a hydroxide ion, the former leading to fixation on the fibre and the latter resulting in hydrolysis of the reactive dye. Scheme 4.3 SD O O CH2CH2OSO3Na SD O O CH CH2 NaOH + Na2SO4 + H2O Sulphatoethylsulphone dye (D = dye chromophore) Vinylsulphone dye SD O O CH CH2 SD O O CH CH2 X SD δ– O δ+ O + X– CH2CH2X+ H+ Vinylsulphone dye Transient species Hydrolysed dye (X = OH) or dyed fibre (X = O–cellulose) Remazol (HOE) dyes, based on the 2-sulphatoethylsulphone precursor of the vinysulphone reactive system (Scheme 4.3) or related species, function by a nucleophilic addition mechanism rather than substitution. Before this can occur, however, alkaline 1,2-elimination of the precursor grouping is necessary to release the reactive vinylsulphone system. In this system the carbon–carbon double bond is polarised by the powerfully electron-attracting sulphone group. This polarisation confers a positive character on the terminal carbon atom, favouring nucleophilic addition of either a cellulosate anion or a hydroxide ion, again leading to either fixation or hydrolysis respectively. Scheme 4.2 N N N NH Cl Cl D N N N NH Cl D X Cl N N N NH X Cl D – Cl–+ X– Dichlorotriazine dye (D = dye chromophore) Transient species Partly hydrolysed dye (X = OH) or dyed fibre (X = O – cellulose)
  • 202. 195 HomeContents The first range of bifunctional reactive dyes containing two distinct reactive systems in the same molecule was the Procion H-E (Zeneca) range, in which the two aminochlorotriazine groupings normally show the same level of reactivity, since these dyes are symmetrical in structure (Figure 4.1). They may be based on one disazo chromophore having a reactive group at each end of the molecule, or on a pair of identical chromophores linked via chlorotriazine units to a central linking diamine. Such approaches have also been exploited in the recent development of the Kayacelon React (KYK) range of neutral-fixing bis(aminonicotinotriazine) dyes. N N N N N N NHD NH D ClCl N H X N H Disazo chromophore (D) RNH2 = arylamine Identical chromophores (D) H2N—X—NH2 = linking diamine N N N N N N NHR NH R ClCl N H D N H Figure 4.1 N N N NHD Cl D = dye chromophore SO2CH2CH2OSO3Na N H Figure 4.2 The past decade has seen the appearance of two further interesting ranges of bifunctional dyes that are capable of reacting with cellulose via both mechanisms, nucleophilic substitution and nucleophilic addition. In both systems one ring substituent in a halogenotriazine dye carries a 2-sulphatoethylsulphone grouping (Figure 4.2). The halogeno substituent can be either chlorine, as in the typical Sumifix Supra (NSK) arrangement shown, or the more reactive fluorine, as in the Cibacron C (CGY) range. REACTIVE SYSTEMS
  • 203. 196 DYEING WITH REACTIVE DYES HomeContents 4.3.1 Monofunctional systems Some of the most important monofunctional reactive systems contain only one possible reactive centre, such as the halogeno substituent in the aminohalotriazine dyes, or the activated terminal carbon atom in the vinylsulphone system. In others there are two equivalent replaceable halogeno substituents, as in the dichlorotriazine, difluoropyrimidine or dichloro- quinoxaline heterocyclic ring systems. When one of these halogen atoms is displaced by reaction or hydrolysis, as in Scheme 4.2 for example, the reactivity of the remaining halogeno substituent is greatly decreased by the presence of the new hydroxy or cellulosyl substituent. Dichloro-s-triazine dyes In a typical dye of this type, such as CI Reactive Red 1 (Scheme 4.1), the partial polarisation of the ring C=N and C–Cl bonds, because of the greater electronegativity of the N and Cl atoms, makes the 2- and 4-chloro substituents susceptible to nucleophilic displacement, although these influences are muted by feedback of electrons from the NH bridging group linking the triazine ring to the phenylazonaphthyl chromophore. Such dyes are stable in neutral solution at ambient temperature but subject to hydrolytic attack by hydroxide ions at alkaline pH and to autocatalytic hydrolysis under acidic conditions. To guard against this, a buffer is added to the solution to ensure stability during isolation and further buffer is added to the dyestuff paste before drying [3]. The dichlorotriazine dyes are highly reactive and can be readily fixed to cellulosic materials by pad–batch dyeing at ambient temperature or by batchwise methods at 30–40°C. This means that relatively small chromogens are preferred to ensure adequate mobility of dye on the fibre during the exhaustion stage. This requirement makes these dyes eminently suitable for bright dyeings but less satisfactory for deep tertiary hues, since the larger-size chromogens used for this purpose often fail to give acceptable performance by low-temperature application. A weakness with certain dichlorotriazine dyes, particularly red dyes based on H acid as coupling component, such as CI Reactive Red 1, is that under conditions of low pH the dye–fibre bond is broken by acid-catalysed hydrolysis, leading to deficiencies in fastness to washing or acid perspiration. When partial hydrolysis occurs as in Scheme 4.2 to form the 2-chloro-4- hydroxy-s-triazinylamino species, the dye does not have a further chance to achieve fixation via the remaining chlorine atom. Under the alkaline conditions of the fixation stage, ionisation of the acidic 4-hydroxy substituent leads to a
  • 204. 197 HomeContents massive feedback of negativity into the triazine ring, causing total deactivation of the remaining 2-chloro substituent. Controlled reaction of dichlorotriazine dyes with either amines or alcohols leads to two further classes of monofunctional dyes, the 2-amino-4-chloro- and 2-alkoxy- 4-chloro-triazines respectively. The latter are more reactive than the former but less reactive than the parent dichlorotriazine types. They are now only of historical interest, the 2-isopropoxy-4-chloro system forming the basis of the Cibacron Pront (CGY) range for printing. The bulky isopropoxy group was chosen in order to disrupt the planarity of the substituted triazine system and thus favour removal of unfixed dye from the printed fabric during the washing-off stage. In a recent investigation, the relative reactivities of model 2-alkoxy-4- chlorotriazine dyes were compared. Surprisingly, the hydroxide ion preferentially displaced the alkoxy group rather than the chloro substituent. Increasing the size and electron-donating capacity of the alkoxy group resulted in a decreasing propensity for substitution, so that displacement of methoxide ion was 12 times faster than displacement of isopropoxide ion [11]. Aminochloro-s-triazine dyes Reaction of a dichloro-s-triazine dye with an amine at about 25–40°C (Scheme 4.1) produces a much less reactive 2-amino-4-chloro derivative as exemplified by CI Reactive Red 3. More energetic reaction conditions, typically 80°C and pH 11 for batchwise application, are necessary for efficient fixation on cellulosic fibres. Early studies of the relationships between structure and substantivity of aminochlorotriazine dyes revealed that the NH bridging groups linking the chromogen and the uncoloured arylamino substituent to the heterocyclic ring had marked effects on the solubility and dyeing properties of the dyes [1]. Replacement of the simple NH imino group by an N-methylimino bridge tended to lower the substantivity for cellulose. The use of a sulphonated arylamine to form the uncoloured 2-arylamino substituent of a monochlorotriazine dye was helpful to enhance solubility and modify the dyeing behaviour. Aminofluoro-s-triazine dyes A fluorine atom is used as the leaving group in the Cibacron F (CGY) range of 2- amino-4-fluoro-s-triazine dyes. The greater electronegativity of fluorine compared with chlorine results in a markedly higher level of reactivity for these dyes than for the 2-amino-4-chloro analogues. The substantivity and solubility of REACTIVE SYSTEMS
  • 205. 198 DYEING WITH REACTIVE DYES HomeContents the dye structures can be modified considerably by careful introduction of appropriate substituents on the chromogen, the uncoloured arylamine and the bridging NH groups (Figure 4.3). Trichloropyrimidine dyes The 1,3-diazine grouping in the pyrimidine ring provides much less activation of chloro substituents than the 1,3,5-triazine system. Fixation to the fibre by batchwise application methods requires treatment at the boil rather than the 80°C found to be optimum for the aminochlorotriazine dyes, but the dye–fibre bond containing a diazine ring is more stable than that containing a triazine nucleus [10]. A study of the relative reactivities of the chlorine atoms in tetrachloropyrimidine with arylamines demonstrated [12] that formation of a trichloropyrimidine dye occurs by nucleophilic substitution of the 4-chloro substituent to form a bridging NH link with the chromophore, as indicated in the structure of CI Reactive Red 17 (Figure 4.4). N HN N OH NaO3S SO3Na N N Cl Cl SO3Na Cl Figure 4.4 This preferential substitution at the 4-chloro position is much less pronounced in the reaction of 2,4,6-trichloropyrimidine with arylamines, so that the dichloropyrimidine dyes formed in this way contain a mixture of 2,6- and 4,6- dichloro isomers. These dyes are even less reactive than the trichloropyrimidine dyes but are correspondingly more resistant to acidic and alkaline hydrolysis. SO3Na SO3Na N HN N OH NaO3S SO3Na N N N NH F Figure 4.3
  • 206. 199 HomeContents The 5-chloro substituent in the trichloropyrimidine reactive system is much less activated by the nitrogen atoms in the heterocyclic ring and is thus not normally capable of hydrolysis or reaction with the fibre. Dyes prepared with a more electronegative substituent in the 5-position, such as 5-cyano or 5-nitro, show enhanced reactivity of the 2,6-dichloro substituents but somewhat lower stability of the dye–fibre bond. Conversely, dichloropyrimidine dyes containing a 5-methyl substituent are less reactive but more stable than trichloropyrimidines [10]. Chlorodifluoropyrimidine dyes Another important route to more reactive halopyrimidine dyes is to use fluorine rather than chlorine in the reactive centres. As with the fluoro-s-triazine dyes, this results in a markedly higher level of reactivity compared with the corresponding chloro-substituted analogues. The Drimarene K (S) and Levafix E-A (BAY) ranges are both based on the 5-chloro-2,6-difluoropyrimidine reactive system (Figure 4.5). Batchwise dyeing temperatures for optimal fixation of these dyes are about 40–50°C. The dye–fibre bond formed by reaction of cellulose with the highly reactive difluoropyrimidine system is more stable to acid conditions than that of the competing dichlorotriazine system but it does tend to undergo oxidative cleavage more readily under the influence of light exposure in the presence of peroxy compounds (see section 4.8). N N OH NaO3S NH SO3Na CH3O N N Cl F FFigure 4.5 Dichloroquinoxaline dyes One of the interesting intermediates exploited in the early years of reactive dye development was 2,3-dichloroquinoxaline-6-carbonyl chloride which could be condensed with the amino group of the dye base to form an amide bridging link between the chromogen and the reactive system [13]. The reactivity of this system is much higher than that of corresponding dichloropyrimidine dyes and comparable with that of the dichlorotriazine and difluoropyrimidine systems, REACTIVE SYSTEMS
  • 207. 200 DYEING WITH REACTIVE DYES HomeContents optimal fixation being achieved by batchwise dyeing at about 50°C. Thus the Levafix E (BAY) dichloroquinoxaline dyes are fully compatible with the Levafix E-A (BAY) difluoropyrimidine members of this composite range. The structure of a typical red Levafix E dye is shown in Figure 4.6. Unlike all other important haloheterocyclic reactive systems, the bridging link between chromogen and reactive grouping is amidic and thus expected to be readily hydrolysed under acidic conditions. The 1,4-diazine ring in the dye–fibre bond formed by these dyes, like the 1,3-diazine ring present after fixation of the difluoropyrimidine system, tends to undergo oxidative cleavage when exposed to light or heat under peroxidic conditions. In spite of these potentially severe defects, the continued commercial success of the Levafix E dyes over a long period suggests that they do not give rise to serious practical problems under normal circumstances [1]. N N OH NaO3S SO3Na SO3Na HN O N N Cl Cl C Figure 4.6 Sulphatoethyl-sulphone and -sulphonamide dyes The Remazol (HOE) vinylsulphone dyes, containing the characteristic 2- sulphatoethylsulphonyl precursor grouping, are intermediate in reactivity between the high-reactivity heterocyclic systems, such as dichlorotriazine or difluoropyrimidine, and the low-reactivity ranges, such as aminochlorotriazine or trichloropyrimidine. Exhaust dyeing temperatures between 40 and 60°C may be chosen, depending on pH, since caustic soda is often selected to bring about alkaline hydrolysis of the precursor sulphate ester. These dyes are applicable by a wide variety of batchwise and continuous processes. The substantivity of many of these dyes is markedly lower than that of typical haloheterocyclic dyes. Not only has the vinylsulphone group, unlike the heterocyclic ring systems, little if any inherent affinity for cellulose, but the terminal sulphato group enhances the aqueous solubility of the precursor form before 1,2-elimination to the vinylsulphone. In contrast to the haloheterocyclic systems, the dye–fibre bonds formed by the vinylsulphone dyes are at their weakest under alkaline conditions [14].
  • 208. 201 HomeContents For three decades the two most widely used reactive dyes have been those illustrated in Figure 4.7: Remazol Black B and Remazol Brilliant Blue R. The four solubilising groups in the precursor form of CI Reactive Black 5 confer high solubility but unusually low substantivity. It is a nearly symmetrical bis(sulphatoethylsulphone) structure and as these precursor groups lose their ionic charge by 1,2-elimination, the substantivity for cellulose is enhanced and the bis(vinylsulphone) structure formed shows excellent fixation efficiency under alkaline conditions. After fixation the inherently low substantivity of the unfixed bis(hydroxyethylsulphone) dye makes washing-off easy in a region of the colour gamut where this is often notoriously difficult [3]. The extremely attractive bright blue hue combined with excellent light fastness of CI Reactive Blue 19 could not be challenged by other reactive blue dyes for many years. The aqueous solubility of this dye is inherently low and depends on the zwitterionic 1-amino-2-sulpho grouping after 1,2-elimination of the sulphate ester has taken place. This has led to poor reproducibility and levelling problems, but nevertheless this dye has remained second only to Black 5 in terms of market share amongst reactive dyes. In a recent investigation of the effect of low-frequency ultrasonic waves on the stability of the copper-complex phenylazo H acid dye Remazol Brilliant Violet 5R (Figure 4.8), reaction rates were recorded for the 1,2-elimination of the sulphato group to form the vinylsulphone and for hydrolysis of the latter to form the 2-hydroxyethylsulphone [15]. Figure 4.7 REACTIVE SYSTEMS N N OH NaO3S SO3Na NH2 NaO3SOCH2CH2O2S NN SO2CH2CH2OSO3Na NH3 HN O O SO3 – SO2CH2CH2OSO3Na CI Reactive Black 5 CI Reactive Blue 19
  • 209. 202 DYEING WITH REACTIVE DYES HomeContents Figure 4.8 N N O NaO3S SO3Na NHCOCH3 NaO3SOCH2CH2O2S O CuII CI Reactive Violet 5 Certain members of the Hoechst range are designated Remazol D brands and these contain a 2-sulphatoethylsulphonamide precursor grouping formed by reacting the dye base with carbyl sulphate [3]. As in the case of the conventional vinylsulphone types, 1,2-elimination occurs under alkaline conditions to give the reactive vinylsulphonamide group and this is capable of either reaction with cellulose or hydrolysis to the hydroxyethylsulphonamide according to the usual nucleophilic addition mechanism (see Scheme 4.3). Phosphonic acid dyes As in the case of chlorotriazines and vinylsulphones, dyes containing phosphonic acid groups had been prepared long before their utility as reactive dyes for cellulose was recognised [4]. The realisation that phosphonic acid derivatives could, under certain circumstances, react with alcohols to give phosphonate mono-esters was made in 1973 at the Stanford Research Institute. Subsequent development work by Burlington Industries and later ICI led to the commercial launch of the Procion T (ICI, now Zeneca) range of dyes in 1977. These dyes were unusual in being applied to cellulosic and blend fabrics under mildly acidic conditions (pH 5–6). This made them particularly suitable for polyester/cotton dyeing by the pad–dry–thermosol process, since the alkaline conditions normally required to fix conventional reactive dyes could be avoided. The structures of many of these dyes were based on the versatile intermediate 3-aminophenylphosphonic acid attached to typical monoazo chromogens in various ways [4]. One example was the 1:2 cobalt complex Procion Rubine T- 6BD (Figure 4.9). These dyes were sold as aqueous solutions of the ammonium salts, which facilitated thermal dissociation to generate the free acid form. This in turn reacted with a carbodiimide, e.g. dicyandiamide, by one of two possible mechanisms, either or both of which may be operative. One mechanism involves formation of an O-acylisourea derivative of the dye phosphonate and the carbodiimide [4], the other proceeds via a dye phosphonic anhydride derivative [16]. Dye fixation can occur by reaction of either of these derivatives with the hydroxy groups of cellulose.
  • 210. 203 HomeContents Figure 4.9 (H4N)2O3P HN N SO3NH4 O N NO2 O N O N O CoIII H4NO3SO2N NH PO3(NH4)2 CI Reactive Red 179 More recently, the role of the carbodiimide in the dyeing process has been examined and it was concluded that the loss of this auxiliary by thermal decomposition was the most important factor limiting dye fixation [17]. Later it was shown that fixation yields as high as 95% could be achieved by minimising the decomposition of the carbodiimide [18]. A major attraction of the phosphonic acid dyes was the exceptional stability of the dye–fibre bonds, more stable than those from chlorotriazine dyes in acidic media and those from vinylsulphone dyes under alkaline conditions. Technical drawbacks of these first acid-fixing reactive dyes, however, included dye migration during drying, especially on heavy fabrics such as corduroy, and strength loss of the cellulosic fibres during thermofixation under the acidic conditions necessary for fixation. Consequently, the Procion T (ICI, now Zeneca) dyes were withdrawn in 1987, essentially for economic reasons [4]. 4.3.2 Bifunctional systems In the early years of the commercial exploitation of reactive dyes, it was soon noted that anomalously high values of cuprammonium fluidity were observed for dyeings of many reactive dyes in full depths, although tests of tensile strength demonstrated that the cellulose remained undamaged [19]. Investigation showed that these anomalous results were associated with those dyes capable of forming crosslinks between neighbouring cellulose chains, such as the bis(sulphato- ethylsulphone) dye CI Reactive Black 5 (Figure 4.7) or the dichlorotriazine dyes with two chloro substituents capable of attack by cellulosate segments of the polymer chains, such as CI Reactive Red 1 (Scheme 4.1). These phenomena attracted further interest when ICI introduced the Procion REACTIVE SYSTEMS
  • 211. 204 DYEING WITH REACTIVE DYES HomeContents H-E dyes, a full range of high-fixation dyes containing two aminochlorotriazine groups per molecule. A detailed study of representative members of this range, as well as other potentially crosslinking reactive systems (dichlorotriazine, chloromethoxytriazine, dichloroquinoxaline, trichloropyrimidine and chlorodifluoropyrimidine) provided convincing evidence of the extent of crosslinking that could take place. The degree of crosslinking was non-existent or relatively insignificant for typical pad–batch dyeings at ambient temperature, but thermal fixation by the pad–dry–steam method resulted in a much higher proportion of crosslinked dye molecules [20]. Recently attention has turned to the more difficult problem of analysing the mode of fixation to cellulose of a bifunctional reactive dye of the Sumifix Supra (NSK) range that contains two dissimilar reactive systems (vinylsulphone and chlorotriazine). Controlled enzymatic degradation of a mechanically milled cotton fabric that had been batchwise dyed at 60°C yielded interesting results [21]: (1) About 80% of the vinylsulphone groups had reacted with the cellulose. (2) About 50% of the chlorotriazine groups had not reacted with cellulose and only half of these had hydrolysed to OH groups. (3) A considerable proportion of the dye molecules had formed crosslinks by reacting via both mechanisms. Bis(aminochlorotriazine) dyes The concept of dyes with two reactive systems of the same type dates back to the early days of reactive dyes but it was not until the early 1970s that the advantages accruing from this arrangement were fully realised in a commercial range of dyes designed in this way. The objective was to produce bifunctional dyes for exhaust dyeing (the major sector, accounting for more than 50% of the total market for reactive dyes) that exhibited substantially higher substantivity, exhaustion and fixation values compared with corresponding monofunctional amino- chlorotriazine dyes. Unlike the sulphatoethylsulphone group, substituted triazine reactive systems readily lend themselves to substantivity enhancement [4]. These bifunctional dye molecules (Figure 4.1) are approximately twice the size of analogous monofunctional types (e.g. CI Reactive Red 3, Scheme 4.1). Their high substantivity allows them to achieve excellent exhaustion at the preferred batchwise dyeing temperature of about 80°C, leading to fixation values of about 70–80%, although typical members of the range, such as Procion Navy H-ER (Figure 4.10), may require salt concentrations as high as 100 g l–1 for optimal yield. High-temperature application conditions ensure good levelling and the high fixation leads to better utilisation of the applied dye, with less hydrolysed
  • 212. 205 HomeContents dye and less discoloration of the effluent. Unfortunately, the rate of removal of unfixed dye at the washing stage is slow owing to the high intrinsic substantivity of these dyes. Bis(aminonicotinotriazine) dyes An aminochlorotriazine dye will react with a tertiary amine possessing a sterically accessible nitrogen atom to produce a quaternary ammonium derivative. The positive charge carried by the quaternary nitrogen atom increases the polarisation of the C–N bond that links it to the triazine ring, so making these compounds much more reactive than the parent dyes. Within a few years of the introduction of reactive dyes, the utility of tertiary amines as catalysts to assist the fixation of aminochlorotriazine dyes to cellulose was evaluated. Suitable tertiary amines include trimethylamine, N,N-dimethylhydrazine, pyridine and the cycloaliphatic compound 1,4-diazabicyclo[2,2,2]octane (DABCO). This approach appeared to offer the opportunity to achieve a range of reactivities between those of aminochloro- and dichlorotriazine dyes without loss of the desirable stability of the dye–fibre bond to acidic conditions that is characteristic of the aminochlorotriazine types. Unfortunately, this ideal was not attainable because of the objectionable odours of the parent tertiary amines liberated by the fixation reaction and the sensitivity of the reactivity behaviour of the quaternised derivatives to the nature of the chromogenic substituent on the triazine ring, making it difficult to select compatible combinations of dyes. Nicotinic acid (3-carboxypyridine) readily dissolves in the dyebath and thus overcomes the odour problem. Advantage of this was taken at ICI in 1979 with the introduction of the bis(aminonicotinotriazinyl)-substituted triphenodioxazine dye Procion Blue H-EG (CI Reactive Blue 187) for exhaust dyeing at 80°C with the other bis(aminochlorotriazine) members of the range. Although this product enjoyed some commercial success it was more reactive than typical Procion H-E Figure 4.10 NaO3S N N N NH Cl NH N SO3Na N OH NaO3S NH2 SO3Na N N NaO3S HN N N N Cl HN SO3Na CI Reactive Blue 171 REACTIVE SYSTEMS
  • 213. 206 DYEING WITH REACTIVE DYES HomeContents dyes, rendering it somewhat incompatible in terms of reactivity profile. Indeed, it was noted that the fixation to cellulose began prior to the addition of alkali to the dyebath. Procion Blue H-EG was eventually superseded by the more conventional bis(aminochlorotriazinyl)-substituted triphenodioxazine analogue Procion Blue H-EGN (CI Reactive Blue 198). The observation that alkali was not essential for fixation of nicotinotriazines to take place was exploited in 1984 by Nippon Kayaku. A full range of bis(aminonicotinotriazine) dyes was introduced, an example being Kayacelon React Red CN-3B, the nicotino analogue of Procion Red H-E3B (Figure 4.11) [22]. Exhaust dyeing is recommended from a neutral bath at temperatures above the boil, conditions that render these dyes suitable, in combination with disperse dyes, for the one-bath dyeing of polyester/cotton blends. By operating at a temperature in the region of 130°C, the diffusion problems expected with such large molecules are minimised [4]. The product of reaction of a bis(aminonicotinotriazine) dye with cellulose is the same as that from the analogous bis(aminochlorotriazine) dye in terms of hue, colour fastness and stability of the dye–fibre bond. If desired, these bis- quaternary dyes can be applied with optimum fixation at 80°C and pH 11, as in the case of Procion Blue H-EG. They have a slightly higher reactivity than vinylsulphone or chlorodifluoropyrimidine dyes, but are less reactive than dichlorotriazine or dichloroquinoxaline systems [23]. The kinetics of quaternisation with DABCO or nicotinic acid and the use of these tertiary amines as catalysts for the high-temperature neutral fixation of the aminochlorotriazine dye CI Reactive Red 3 (Scheme 4.1) and its bis(amino- Figure 4.11 SO3Na N N OH NaO3S SO3Na N N NHN X HN N N N NH X NH OH NaO3S SO3Na N N NaO3S CI Reactive Red 120 Red 221 Substituent X chloro nicotino Procion Red H-E3B (Zeneca) Kayacelon React Red CN-3B (KYK)
  • 214. 207 HomeContents chlorotriazine) analogue Red 120 (Figure 4.11) have been investigated recently [22]. A series of bis-quaternary derivatives of CI Reactive Red 120, including Red 221 as a control, have also been synthesised using trimethylamine, DABCO, pyridine, nicotinamide (pyridine-3-carbonamide) and isonicotinic acid (4- carboxypyridine), as well as nicotinic acid for the control dye. All these dyes performed well in neutral or alkaline dyeing conditions with the exception of the nicotinamido derivative at alkaline pH or at 130°C and pH 7; this was attributed to nucleophilic attack by hydroxide ions leading to ring opening to form an aminotriazine [24]. Aminochlorotriazine-sulphatoethylsulphone dyes Reaction of a dichlorotriazine dye with an arylamine containing a 2- sulphatoethylsulphone grouping is the preferred route to mixed bifunctional reactive dyes capable of reacting with cellulose either via a monochlorotriazine moiety or a vinylsulphone group. A typical example is Sumifix Supra Brilliant Red 2BF (NSK) shown in Figure 4.12 [25]. Both reactive systems can contribute to the fixation process but the relatively greater reactivity of the vinylsulphone group ensures that most of the fixation arises via this function [21]. Further benefits of this type of structure, however, include the higher substantivity contributed by the triazine bridging residue and the capability this gives to link a sulphatoethylsulphone grouping to a wide range of chromogens. The presence of two reactive groups that differ in reactivity gives dyes that are less sensitive to exhaust dyeing temperature than typical monofunctional reactive dyes. They can be applied over a wider range of temperatures (50–80°C) and reproducibility of hue in mixture recipes is improved. Moreover, they show minimal sensitivity to electrolyte concentration and are less affected by changes in liquor ratio [26]. Low dyeing temperatures favour reaction via the vinylsulphone group and at higher temperatures the contribution of the chlorotriazine system to fixation becomes more important [27]. REACTIVE SYSTEMS Figure 4.12 SO3Na N N OH NaO3S SO3Na N N NHN Cl HN SO2CH2CH2OSO3Na CI Reactive Red 194
  • 215. 208 DYEING WITH REACTIVE DYES HomeContents The presence of two types of dye–fibre bond has certain consequences for fastness properties. Mixed bifunctional dyes show superior fastness to acid storage than dichlorotriazine or dichloroquinoxaline systems and better fastness to peroxide washing than difluoropyrimidine or dichloroquinoxaline dyes [28]. Conversely, mixed bifunctional dyes do show some of the weakness to alkaline treatments characteristic of vinylsulphone dyes, although less so than monofunctional dyes relying solely on these groups for fixation [29]. An admirable investigation of 18 isomeric dyes of the 7-phenylazo-N- (anilinochlorotriazinyl) H acid series, including CI Reactive Red 194 (Figure 4.12) and two other commercial products, was reported recently [25]. All possible combinations of o-, m-, p-orientation of the sulpho and sulphato- ethylsulphonyl groups in the phenylazo and anilino rings at opposite ends of the molecule were evaluated. Interestingly, the three commercially utilised structures showed the highest fixation yields amongst the series of 18 dyes studied. Aminofluorotriazine-sulphatoethylsulphone dyes Early in 1988 Ciba-Geigy launched the Cibacron C range of mixed bifunctional dyes. They contain a new aliphatic vinylsulphone system and either a monofluorotriazine bridging group (Figure 4.13) or an arylvinylsulphone function [30]. They are designed mainly for pad applications and appear to be characterised by medium to low affinity, good build-up, easy wash-off and high fixation. Their outstanding bath stability and high fixation make them especially suitable for pad–batch dyeing [31]. The manufacturing cost of these structures is believed to be relatively high but the purchase cost to the dyer may be offset by enhanced cost-effectiveness in use attributable to efficient fixation and easy wash-off, possibly the best approach so far towards environmentally acceptable reactive dyes [4]. An important feature of the Sumifix Supra (NSK) type of bifunctional system (Figure 4.12) is the major difference in reactivities between the amino- N N N NHD F NH R SO2CH2CH2OSO3Na D = dye chromophore R = aliphatic group Figure 4.13
  • 216. 209 HomeContents chlorotriazine moiety and the much more reactive vinylsulphone group. There are some practical conditions, notably in pad–batch application, that do not allow full advantage to be taken of both types of reactive group present. The combination of aminofluorotriazine and vinylsulphone in the Cibacron C (CGY) system, both groups offering effective fixation under virtually the same conditions, exploits the concept of bifunctionality more effectively. These factors have been demonstrated elegantly in a detailed evaluation by pad–batch dyeing of cotton with three commercially important copper formazan reactive dyes: (1) Cibacron Blue F-R (CI Reactive Blue 182) with a monofunctional 2-fluoro- 4-sulphoanilinotriazine system. (2) Sumifix Supra Blue BRF (CI Reactive Blue 221) with a mixed bifunctional 2-chloro-4-vinylsulphonylanilinotriazine system. (3) Cibacron Blue C-R with a synchronised bifunctional 2-fluoro-4-vinyl- sulphonylalkylaminotriazine system. Under the relatively mild conditions of pad–batch fixation within 6 h batching time, the mixed bifunctional system behaved more like a monofunctional vinylsulphone dye and only the synchronised Cibacron C (CGY) system showed truly bifunctional performance [32]. 4.4 STRUCTURE AND PROPERTIES As already noted in the preceding section, the four characteristic features of a monofunctional reactive dye are the chromogen, the reactive system, the bridging link and at least one (but usually more) solubilising group. In a mixed bifunctional structure there are at least two of these except the chromogen, and in symmetrical bis(aminochlorotriazine) or bis(aminonicotinotriazine) dyes there may be twin chromogens (Figure 4.11). The design of reactive dye structures almost always involves one or more compromises between conflicting requirements. There is seldom an ‘ideal’ structure of a desired hue that embodies all possible attractive features with regard to application and fastness properties. The gain in aqueous solubility provided by an extra sulpho group often has to be paid for by a decrease in affinity for cellulose. Enhancement of substantivity is beneficial for high exhaustion but may impair migration or washing-off characteristics. High reactivity offers the possibility of rapid fixation but storage stability may be adversely affected. STRUCTURE AND PROPERTIES
  • 217. 210 DYEING WITH REACTIVE DYES HomeContents 4.4.1 Chromogens in reactive dye structures Providing there are enough sulpho groups to ensure adequate solubility in water, the only essential feature of a chromogen needed to incorporate it into a reactive dye molecule of the haloheterocyclic type is a primary or secondary amino group to which the heterocyclic bridging group can be attached. This also applies to the various ranges of bifunctional systems described in section 4.3.2, since they all contain this same type of amino-s-triazine bridge attached to the chromogen. Only in the case of the monofunctional sulphatoethylsulphone dyes is the selection of chromogens more limited by the various ways in which intermediates such as m- or p-(2-sulphatoethylsulphonyl) aniline derivatives (see, for example, Figure 4.7) can be exploited as an integral part of the chromogenic grouping [3]. Greenish yellow reactive dyes are invariably monoazo types with a heterocyclic coupling component and the reactive system usually located in the diazo component, whereas reddish yellows are usually arylazoanilines with the reactive group on the anilino ring. A recent study of the photochromism of such dyes demonstrated that they tend not to be photochromic if they can form a hydrazine tautomer [33]. Typical orange and scarlet reactive dyes are monoazo derivatives of J acid (Figure 4.5) and in bright bluish reds the all-important coupling component is H acid (see, for example, Figure 4.3 or 4.11). Copper- complexes of 2-aminophenol derivatives, diazotised and coupled with J acid for rubines (Figure 4.9) or H acid for violets (Figure 4.8) dominate the duller and bluer reddish hues. Twice-coupled H acid chromogens (Figure 4.10) predominate in the navy blue sector because of their economic advantages. The brighter royal blues were traditionally the province of anthraquinone dyes such as Remazol Brilliant Blue R (Figure 4.7) but in recent years the triphenodioxazine dyes such as Procion Blue H- EGN have gained an important share of these hues because they are tinctorially much stronger and hence economically advantageous. Turquoise blues remain totally dominated by copper and nickel phthalocyanine derivatives [3]. 4.4.2 Structure and substantivity of reactive systems The important influence on substantivity of the reactive system itself was recognised in the early years of the development of reactive dyes [10]. The dichlorotriazine group and even more so the aminochlorotriazine or dichloroquinoxaline systems considerably enhance the overall substantivity of the dye. The pyrimidine-based systems, with one less heterocyclic nitrogen atom than the corresponding triazine derivatives, contribute much less to the total
  • 218. 211 HomeContents substantivity of the structure. The non-aromatic reactive systems, notably the 2- sulphatoethylsulphone group, modify the substantive effect of the chromogen only slightly or not at all. This can offer a significant practical advantage, since a low overall substantivity that remains unaffected by the reactive system promotes completeness of removal of the unfixed dye on rinsing. 4.4.3 Factors governing reactive dye uptake All conventional reactive dyes for cellulose, irrespective of whether they react by nucleophilic addition, substitution, or both mechanisms (section 4.3), rely on the reactivity of the cellulosate anion as the nucleophilic reagent and hence hydrolysis of the dye by reaction with hydroxide ions from water will always compete with the desired fixation reaction. Reaction between the dye and cellulose can occur only when the dye has been absorbed into the cellulose phase. Thus the kinetics of the dye–cellulose reaction are strongly influenced by the rate of absorption of dye. The ratio of the rate constants for reaction of the dye with the fibre and with water is a constant for a given dye over a wide range of alkaline pH values. The efficiency of fixation is a function of: (1) the reactivity ratio, the ratio of rate constants for the fixation reaction and hydrolysis; (2) the substantivity ratio, the relative concentrations of dye absorbed into the substrate and remaining in the dyebath; (3) the diffusion coefficient of the dye in the substrate; (4) the liquor ratio; and (5) the surface area of the substrate available for absorption of dye [34]. The lower the linear density of the fibre, i.e. the greater the surface area per unit weight, the more efficient is the dyeing. The substantivity ratio is the most influential of the factors governing fixation efficiency. Dyes of higher substantivity diffuse more slowly than less substantive dyes. Changes in dyebath conditions that increase substantivity tend to decrease the diffusion coefficient. Lowering the liquor ratio favours increases in the rate and efficiency of fixation. The full effects of this are never completely realised, however, because the higher dyebath concentration necessary at the lower liquor ratio implies a decrease in substantivity ratio, offsetting some of the expected gain. Substantivity ratio remains approximately constant within the pH range 7–11 at a given electrolyte concentration, but above pH 11 there is a marked fall in substantivity, especially with highly sulphonated dyes. As the applied con- STRUCTURE AND PROPERTIES
  • 219. 212 DYEING WITH REACTIVE DYES HomeContents centration of dye is increased at constant electrolyte concentration, the substantivity ratio and hence the efficiency of fixation are lowered. Thus full- depth dyeings require longer for completion of the reaction and the percentage fixation is usually inferior. In order to attain the maximum rate and efficiency of fixation, more electrolyte is needed, but this increases the risk of aggregation and possible precipitation with dyes of limited solubility. An increase of dyeing temperature lowers the substantivity ratio and accelerates the rate of hydrolysis of the dye; both of these effects reduce the fixation efficiency. The rates of diffusion into and reaction with the fibre are also accelerated, however, and these factors both favour fixation of the dye. An increase in electrolyte concentration always enhances substantivity without impairing reactivity providing the dye remains completely dissolved. The beneficial effects of electrolyte addition are most evident with the more highly sulphonated dyes at relatively high pH and applied depth. Informative studies of the relationships between dye structure and substantivity [35] and between dye structure and levelling properties [36] are available. The interaction between these dyeing properties and the controlling parameters in exhaust dyeing with reactive dyes, such as applied concentration, pH, temperature and electrolyte addition, is the key to achieving successful and reproducible dyeings. A valuable classification of reactive dye types has been formulated recently [37]. Three groups relating to the most important control parameter in each case may be distinguished. Group 1 Alkali-controllable reactive dyes These dyes have optimal temperatures of fixation between 40 and 60°C. They are characterised by relatively low exhaustion in neutral salt solution before alkali is added. They have high reactivity and care in addition of alkali is necessary to achieve level dyeing, preferably at a controlled dosage rate. Typical examples of dyes belonging to this group have dichlorotriazine, chlorodifluoropyrimidine, dichloroquinoxaline or vinylsulphone reactive systems. Group 2 Salt-controllable reactive dyes Dyes in this group show optimal fixation at a temperature between 80°C and the boil. Such dyes exhibit comparatively high exhaustion at neutral pH, so it is important to add salt carefully to ensure that dyeings are level. Electrolyte addition is often made portionwise or preferably at a controlled rate of dosage.
  • 220. 213 HomeContents Dyes with these properties typically have low-reactivity systems such as trichloropyrimidine, aminochlorotriazine or bis(aminochlorotriazine). Amino- fluorotriazine dyes in the Cibacron F (CGY) range have high substantivity and should thus be regarded as salt-controllable but they are sufficiently reactive for fixation at 60°C or even lower temperatures by batchwise application. Group 3 Temperature-controllable reactive dyes This group is represented by those dyes that react with cellulose at temperatures above the boil in the absence of alkali, although if desired they can be applied under the same conditions as the salt-controllable group with alkaline fixation at a temperature between 80°C and the boil. Dyes in this group have self-levelling characteristics so there is no need to use auxiliary products to facilitate level dyeing. Good results can be achieved by controlling the rate of temperature rise. At present only the Kayacelon React (KYK) range of bis(amino- nicotinotriazine) dyes belong to this group. 4.4.4 Physical form and handling of reactive dyes All reactive dyes are prone to hydrolysis in the presence of moisture, especially the highly reactive ranges, and they will deteriorate unless carefully handled and stored. Cool, dry conditions are essential and the lids of packages must be firmly replaced after use. Since reactive dyes in powder form may release dust when disturbed, it is always possible for respiratory allergies to arise with some workers who handle them [38]. For this reason suitable dust-excluding respirators should be used and weighing or dissolving procedures should be carried out in ventilated enclosures. Conventional dye powders are usually dissolved by one of the following techniques: (1) Pasting with cold water followed by the steady addition, with stirring, of the required amount of water at the correct temperature. (2) Sprinkling a steady stream of dye powder into the vortex formed by the high-speed stirring of water at the correct temperature. Few ranges of reactive dyes require boiling water, although Remazol (HOE) vinylsulphone dye powders are dissolved in boiling water followed by passing immediately through a fine sieve into the required volume of cold water. Highly reactive dyes, such as dichlorotriazine or chlorodifluoropyrimidine types, require water temperatures no higher than 50°C. Most dyes of lower reactivity, e.g. STRUCTURE AND PROPERTIES
  • 221. 214 DYEING WITH REACTIVE DYES HomeContents aminochlorotriazine or trichloropyrimidine systems, require a dissolving temperature of 80°C. The dusting problem with some reactive dye powder brands can be avoided by working with granulated or liquid formulations. A recent improvement that ensures troublefree weighing and handling of small amounts for batchwise dyeing has been the development of cold-dissolving granular brands such as the Drimarene CDG (S) dyes [39–43]. These are non-dusting, free-flowing grains that dissolve readily in cold water and offer ease of handling in automatic dissolving and metering devices. The marked tendency of reactive dyes to undergo hydrolysis in solution has delayed the development of liquid formulations until recent years. For continuous dyeing and printing, however, especially where automated metering equipment is installed, liquids are particularly convenient. Liquid brands of the relatively stable types, such as sulphatoethylsulphones, aminochlorotriazines and bifunctional dyes containing both of these systems [44], are well established in commercial use. They are essentially isotropic aqueous solutions of the dyes, often with auxiliaries such as a buffer, a hydrotropic agent such as urea or caprolactam, and often a polymeric stabiliser to inhibit settling out on storage [4]. 4.5 BATCHWISE APPLICATION In spite of the essential simplicity of reactive dyeing methods, there are few instances where dyes based on one type of reactive system show fully satisfactory compatibility with those of another type. Even if competing dyes have identical chromophoric groupings, there may well be marked differences in application behaviour and fastness properties of the resulting dyeings. In selecting a range of reactive dyes for batchwise dyeing, however, the dyer has considerable freedom of choice since almost all required hues are fully represented in every range. Selection is largely based on application technique and consideration of the end use of the material. Reactive dyes can be applied by any conventional batchwise dyeing method for cellulosic materials, including circulating-liquor machines for loose stock, yarn or woven fabrics, as well as jets, winches or jigs for piece dyeing. The conventional dyeing process entails three stages: (1) Exhaustion from an aqueous bath containing electrolyte, normally under neutral conditions. (2) Addition of alkali to promote further uptake and chemical reaction of absorbed dye with the fibre at the optimal pH and temperature. (3) Washing of the dyed material to remove electrolyte, alkali and unfixed dye.
  • 222. 215 HomeContents Numerous variants of this basic procedure in terms of chemical concentrations and fixation conditions have been devised to take account of the characteristic properties of the numerous ranges of reactive dyes now available. Considerable research over several decades has led to the present selection of processes to apply each range of dyes in such a way that optimal fixation efficiency and level dyeing are achieved. In recent years these studies have been concerned particularly with dyes that contain bifunctional reactive systems. For example, a detailed investigation of levelling characteristics has compared a bis(vinylsulphone) and a bifunctional aminochlorotriazine–vinylsulphone with several conventional monofunctional vinylsulphone dyes [36]. Factors controlling the rates of exhaustion and level dyeing behaviour have been analysed to define optimised conditions recommended for batchwise dyeing with Procion H-EXL (Zeneca) bis(aminochlorotriazine) dyes [45] or with Sumifix Supra (NSK) aminochlorotriazine–vinylsulphone dyes [46]. 4.5.1 Preparation and chemicals required Except in special instances, batchwise preparation before reactive dyeing is carried out in the dyeing machine itself or in equipment of similar design reserved for the preparation stage. The essential requirements are that the material must be made available for dyeing in a neutral, uniform and readily absorbent state. In contrast to the application of vat or sulphur dyes, typical reactive dyeing processes will not eliminate natural or added fats and waxes. Acceptable results are often possible on knitgoods without lengthy pretreatment, as in the pad–batch or hot batchwise dyeing processes in the presence of a powerful wetting agent. Residual size must always be removed from woven goods because of the risk of dye wastage by reaction with hydroxy groups in size components. Owing to the brilliant hues of many reactive dyes, sufficient brightness may be attainable on woven cotton without prebleaching. Thoroughly desized and scoured fabrics can be used in many cases. Where bleaching is necessary it is imperative to check that all traces of residual chlorine or peroxy compounds are removed prior to dyeing, otherwise loss of reactivity and even partial destruction of some dyes can occur. There is considerable variation in the ability of reactive dyes to cover dead or immature cotton. For this reason it may sometimes be necessary to causticise or mercerise woven fabrics in order to achieve a satisfactory appearance in certain hues. Such pretreatments give the further advantage of better colour yield. With few exceptions, reactive dyes have good solubility in water. Although seldom sensitive to neutral hard water, precipitation of hardness constituents BATCHWISE APPLICATION
  • 223. 216 DYEING WITH REACTIVE DYES HomeContents results at the alkaline fixation stage and thus soft water should be used for all dissolving and dyebath operations. Sources of water with variable bicarbonate content, as might arise in a water supply with temporary hardness, can adversely affect the reproducibility of the dye fixation conditions. The influence of the bicarbonate anions largely depends on the alkali used for fixation. Alkalis based on sodium hydroxide exhibit higher sensitivity than sodium carbonate systems [47]. The use of either common salt (sodium chloride) or Glauber’s salt (hydrated sodium sulphate), in large amounts, is essential to all batchwise dyeing processes for reactive dyes. The relative prices, purity and availability of these electrolytes vary considerably in different parts of the world and selection must take account of this. Common salt is widely used, but Glauber’s salt is preferred with certain bright royal blues based on anthraquinone chromogens and turquoise or green hues dyed with copper or nickel phthalocyanine derivatives. Common salt is more soluble and easier to dissolve than Glauber’s salt. Electrolyte, from whatever source, must be free from alkali, since the latter causes premature fixation or hydrolysis of the dye. The impurities that can significantly influence the reproducibility of the dyeing process are the alkaline earth metals calcium and magnesium, as well as the transition elements copper and iron. Their adverse effect on the process can be seen as inferior reproducibility, unlevel dyeing and lower wet fastness. The quality and source of electrolyte have a major influence on the levels of trace metal impurities introduced into the dyebath. Common salt is obtained from underground deposits or from seawater. The levels of metal ion impurities vary according to source and degree of purification [47]. These impurities can be controlled by the appropriate use of sequestering agents, taking into account the pH and temperature of the dyeing process. Uncontrolled use of organic sequestrants, such as ethylenediaminetetra-acetic acid, can lead to problems of hue change and lower light fastness. All metal-complex reactive dyes, with the exception of the phthalocyanine derivatives, will give rise to this effect. Whilst soda ash (98% anhydrous sodium carbonate) remains the most widely used alkali for reactive dyeing, sodium bicarbonate, sodium silicate, caustic soda and various phosphates are also important. Alkalis of high purity are recommended and additions of solid brands should be well-diluted beforehand. Hot, damp conditions of storage should be avoided and dry scoops should be used when weighing. Sodium bicarbonate should be dissolved at low temperatures and direct heating by steam injection must be avoided. Caustic soda and sodium silicate are normally marketed and used as concentrated liquors of known concentration. In recent years, liquid buffer systems and liquid alkali
  • 224. 217 HomeContents products such as Alkaflo (Tanatex) have been introduced mainly for automatic dosing and metering systems [48]. Provided goods have been prepared efficiently for exhaust dyeing, it is unnecessary to add wetting or levelling agents to the dyebath. In winch or overflow dyeing of tubular-knitted cotton, however, minimal addition of a wetting agent can provide a lubricating action for the avoidance of rope marks. After any pretreatment with wetting agent, thorough rinsing is advisable before a fresh bath is set for dyeing, in the interest of reproducibility. Unwanted foam in jet or overflow machines can be minimised by careful addition of selected antifoam agents. When dyeing in enclosed machines at temperatures higher than 70°C, certain azo reactive dyes may undergo reduction owing to the combined effects of heat, alkali and aldehydic groups in the cellulose. If this problem is expected to occur, it is advisable to add 1–2 g l–1 of sodium m-nitrobenzene- sulphonate as a protecting agent. 4.5.2 Package dyeing of yarn Reactive dyes are well established for dyeing all types of cotton materials, including sewing threads, in circulating-liquor machines as loose stock, yarn packages or beams of woven fabric. These units are controlled by fully or semi- automatic systems, wholly or partially programmed to regulate temperature, process time, flow-direction cycles, dyebath additions, dosage rates and sequence changes. Machine functions and substrate form (type of fibre, uniformity and accessibility) determine not only the quality of dyeing but also the dyeing procedure. The more thorough the access of dye liquor to the material, the better the levelness and penetration. Rapid dyeing cycles require uniform liquor circulation, which in turn depends on the efficiency of machine loading and the quality of package winding. The latter is determined by angle of wind, tension and material density, and it must allow uniform and consistent flow of liquor. If circulation is poor or non-uniform, good-quality dyeings might still be possible but relatively long dyeing cycles would be necessary, particularly at the critical stages of levelling in salt solution and alkali dosage. If feasible, chemical additions should be made when the liquor is circulating from outside to inside. Special care is necessary with yarns for plain weaving or knitting, as opposed to effect threads in a coloured woven or knitted design. Where penetration problems are anticipated, as in certain mercerised high- twist or multi-ply yarns or in most narrow fabrics, it is advisable to use low- reactivity dyes at temperatures above 70°C, withholding the alkali addition until BATCHWISE APPLICATION
  • 225. 218 DYEING WITH REACTIVE DYES HomeContents at least 30 min after the final addition of salt. When dyeing such materials as webbing, braid or zip-fastener tape, levelling in neutral salt solution can be improved by raising the temperature to the boil or preferably 120°C, before cooling to the optimal temperature for the alkaline fixation step. This technique is not always applicable and all copper-complex dyes are unsuitable, except the copper phthalocyanine derivatives. A reduction inhibitor should be present from the start of dyeing. Conventional package-dyeing machines for cotton yarn operate at a liquor ratio between 10:1 and 15:1. By comparison, short-liquor processing equipment is designed for use at liquor ratios in the range of 4:1 to 6:1. The flow rate, i.e. the amount of liquor pumped through the material, depends on the resistance to flow of the substrate and the efficiency of the pump. The contact number, or rate of exchange of liquor, is one of the most important parameters in yarn dyeing [49]. In short-liquor dyeing the total liquor volume passes through the substrate mass much more frequently than in conventional equipment. The higher the contact number, the better the conditions for level dyeing. Short-liquor processing offers the possibility of tolerating faster rates of heating and cooling. A short liquor ratio can be achieved either by lowering the liquor level in the vessel (sump dyeing) or by filling the chamber as full as possible with yarn packages. In the sump method, the packages are not immersed completely in the dyebath and dye liquor can only be pumped in one direction (out-to-in). This has no adverse effect on the quality of the dyeings but packages must be correctly prepared and leakages avoided. This is achieved using large cylindrical packages wound on special tubes, fitted together and compressed into a continuous column in a hydraulic press [50]. In order to introduce short-liquor methods successfully in yarn processing other developments must be considered, such as: (1) automatic dosing techniques; (2) adjustable flow of circulation pumps; (3) synchronised dyeing; (4) electronic control (section 4.5.8). In synchronised dyeing systems for yarn packages the exchange of dye liquor is constantly adjusted throughout the process. The extent of the adjustment is dependent on substrate, substantivity and processing conditions. Experience confirms that a considerable improvement in the physical quality of the dyed yarn can be achieved [49]. The increasing trend in package dyeing is towards high- volume pressed packages weighing 1.2–2 kg per package. These can often be used directly, without rewinding, in automatic weaving and knitting machines.
  • 226. 219 HomeContents 4.5.3 Jig dyeing of woven fabrics In spite of the outstanding growth in pad–batch dyeing with reactive dyes, jig machines are still widely used for woven cotton. They pose a special problem for reactive dyeing in that marked variations in temperature can exist between the dyebath and the exposed edges of the fabric rolls. Such differences result in ‘listed’ selvedges that are either paler or off-tone relative to the remainder of the dyed fabric. ‘Ending’ can also be apparent because the draw roller (initially at ambient temperature) tends to abstract heat from the adjacent layers of fabric. The equilibrium uptake of dye is not greatly affected but the dye–fibre reaction rate is appreciably reduced at lower temperatures. The use of high-reactivity dyes with medium to high substantivity is the most effective solution to this problem,. Dyebath temperatures of 30–40°C are sufficiently close to ambient conditions, resulting in negligible temperature differentials throughout the goods even under the most adverse conditions. Such dyes exhibit good build-up on the jig at operating liquor ratios in the range 3:1 to 6:1. Pad–jig development gives improved surface appearance and penetration with difficult fabrics. A high pick-up should be avoided to reduce the likelihood of seepage defects. The padded cloth may be passed directly to the jig, set with the full amount of salt, for immediate development. Alternatively, quality can be improved by holding on the turning batching roll for about 30 min before passage to the prepared jig. 4.5.4 Winch and jet dyeing When reactive dyes were first introduced for dyeing knitted and lightweight woven fabrics suitable for processing in rope form, these materials were dyed at long liquor ratios in the range 20:1 to 30:1 on simple winch machines. These lacked liquor circulation or separate addition tanks and poor reproducibility of liquor ratio and temperature was unavoidable. Particular care was necessary to avoid damage during preparation and dyeing, e.g. abrasion, rope marks, ‘crowsfoot’ creasing and running streaks. Disadvantages often associated with reactive dyeing on the traditional winch included: (1) insufficient agitation of the fabric; (2) variable liquor ratio; (3) lack of control of temperature rise; (4) imprecise temperature of fixation; (5) high energy consumption for dyeing and washing-off; (6) large amounts of chemicals required; BATCHWISE APPLICATION
  • 227. 220 DYEING WITH REACTIVE DYES HomeContents (7) large volumes of waste dye liquors; (8) high degree of manual operation. The introduction of temperature regulation was particularly important for reactive dyeing because it was the first essential step towards adequate control of the process. It led to the decline of high-reactivity dyes in long-liquor dyeing and the development of improved low- and moderate-reactivity ranges of more substantive dyes that followed a more favourable exhaustion–fixation profile. The discovery and rapid exploitation of the pressure jet dyeing machine for polyester fabrics in the late 1960s led to the introduction of atmospheric jet machines for the reactive dyeing of cotton knitgoods during the next decade. The pump and venturi jet provided effective liquor circulation and these machines were equipped with time–temperature controllers, a separate addition tank and mechanical features to minimise creasing problems. It was possible to operate at lower liquor ratios in the range 8:1 to 12:1 and to achieve improved liquor flow and level dyeing. These developments promoted liquor exchange so effectively that differences in temperature and concentration within the system became negligible, making it possible to reduce the liquor ratio further and satisfy demands for a more cost- effective process. This reduction of the liquor ratio brought two problems, however: manual addition of the salt to control levelling and manual addition of alkali to bring about fixation [51]. There were restrictions on the addition of salt during running, so the salt-at-start technique became established. The development of metering technology then facilitated accurate control of the reaction at constant temperature. A direct result of short-liquor application is to enhance substantivity at a given salt concentration, or to attain the same substantivity with less salt. A liquor ratio of 5:1 is regarded as the effective minimum for conventional short-liquor dyeing. It is limited by the solubility of the dye, the levelness attainable and the availability of liquor for dissolving and addition of chemicals. The more rapid and higher exhaustion that occurs at these concentrations brings potential problems of unlevelness, owing to various causes: (1) Dyebath exhaustion reached too rapidly (uneven uptake). (2) Excessive exhaustion (inefficient levelling). (3) Solubility of dye exceeded (uncontrolled surface deposition). These risks can be countered by improved control of temperature and rate of salt addition, extending the duration of the migration stage before the controlled addition of alkali [52].
  • 228. 221 HomeContents In fabric rope-dyeing equipment the decrease in liquor ratio has not stopped at the conventional limit of 5:1. As the development of the Then Airflow and the Thies Rototherm machines demonstrate, liquor ratios as low as 3:1 are attainable by rotating the goods in a dyebath-free chamber using continuous impregnation and recirculation of excess dye liquor leaving this chamber. This method is known as the ultra-low liquor ratio (ULLR) technique. The dye–fibre reaction thus takes place from a dyestuff concentrate. This implies a high initial rate of fixation and a correspondingly high risk of unlevelness. The ULLR method thus requires a lower starting temperature, low-substantivity dyes, minimum electrolyte levels and pre-buffering to a low pH. Two-stage addition of the salt minimises the risk of precipitation of the dye at the excessive concentration unavoidable in the initial stage. For most reactive systems the economic benefits claimed for the ULLR technique are difficult to confirm [51]. 4.5.5 Garment dyeing This has become increasingly important in recent years. This trend arose in the 1980s because of the greater dependence of retailers on quick change in colours to meet their need for rapid fashion response. Garment dyeing meets these requirements and lead times have been reduced to one week, as against several weeks with other processing routes [53]. Demand for leisurewear garments has grown and such garments are highly colour- and fashion-oriented. Garment dyeing also offers reduced stockholding and improved monitoring of stocks in fashion colours. Four categories of garment dyeing are important: (1) Fully fashioned garment dyeing. (2) Cut and sew garments dyed to high fastness. (3) All-cotton ‘boutique’ garments dyed to relatively low fastness. (4) Desizing, bleaching and after-washing of yarn-dyed denim. Traditionally garments were dyed with little agitation and slow liquor interchange in paddle machines at liquor ratios of 25:1 to 40:1. Although suitable for loosely knitted fully fashioned garments these conditions give problems in seams or thick areas of heavier garments. The advent of energy- saving rotary dyeing equipment in the 1970s operating at liquor ratios close to 10:1 eventually allowed the dyeing of a much wider variety of fashionable garment constructions. Modern versions of such machines with integral centrifuging and short processing cycles lend themselves ideally to the quick turnaround times now seen as essential [53–55]. Cotton garment dyeing is particularly suited to the production of the ‘totally BATCHWISE APPLICATION
  • 229. 222 DYEING WITH REACTIVE DYES HomeContents relaxed’ fashion look associated with leisurewear and the ‘distressed’ look typical of after-washed denim goods. The last decade has seen periods of high demand for such merchandise. Fashion trends are dynamic, however, and developments have also been undertaken for markets and end uses where a ‘neat’ or ‘pristine’ appearance is demanded [53,56]. A notable advantage of machinery incorporating centrifugal action is the efficient rinsing action in a short time. When dyeing cotton with reactive dyes the washing-off stages can be decreased from about 60 to 10 min, drastically reducing overall process time. Control of all machine functions and process parameters may be accurately regulated automatically. Systems are available whereby the dyeing machine may be controlled from a central console, including automatic additions of dyes and chemicals from a central dispensing area. The requirements of a reactive dye range for cotton garment dyeing include: (1) good level dyeing properties; (2) rapid migration and diffusion properties to assist seam penetration; (3) reproducibility of hue in combination recipes; (4) suitability for automated rapid processing. The higher the reactivity of the dye system, the lower the application temperature and the higher the rate of fixation at alkaline pH. The above requirements are met by low-reactivity dyes with a high temperature of fixation so that diffusion and migration are optimised. Linear addition profiles for both dye and alkali additions are preferred to assist level dyeing. Rinsing and soaping are favoured in rotary-drum machines using the hydroextraction facility that greatly improves liquor interchange between rinses. Problems and faults that may arise in garment dyeing have been outlined [57– 59]. These include shrinkage, surface abrasion, peeling or linting, creasing, deformation, unlevel dyeing and poor penetration of seams. Trimmings, buttons, zips and other accessories may give special difficulties. The design of such garments, particularly sportswear and other stretchable constructions, must envisage and provide for the garment dyeing stage. This requires closer links between designers, manufacturers, finishers and retail organisations. The fabric should be designed, manufactured and prepared with processing shrinkage in mind and the inclusion of seams, sewing thread, zips and other accessories planned to minimise potential problems at the dyeing stage. 4.5.6 Dyeing of regenerated cellulosic materials Exhaustion and fixation of reactive dyes on viscose or modal fibres are normally
  • 230. 223 HomeContents higher than on cotton or linen, but this is not generally true for polynosic or high-tenacity fibres. The substantivity of individual dyes for these less amorphous regenerated fibres is somewhat more variable. Many reactive dyes show significantly higher fastness to light on viscose than on cotton at equivalent applied depths. The recent development of viscose micro-fibres has brought reflectance values and colour yields closer to those of cotton, improving solidity in blends [60,61]. Lightweight viscose materials can be dyed at relatively low temperatures with high-reactivity dyes using processes that are not much different from corres- ponding methods for cotton. Yarns and fabrics made from viscose fibres of larger diameter or heavyweight fabric constructions may show poor penetration and surface frostiness under these conditions, however. Dyes of low to moderate reac- tivity requiring fixation temperatures of 60°C or higher give better results on materials that give inferior levelling or inadequate penetration at lower temperatures. Salt and alkali requirements are generally lower for reactive dyes on viscose than for the corresponding dyeings on cotton but most turquoise and green hues based on phthalocyanine derivatives are applied according to special recom- mendations. The pad–jig development process outlined for cotton (section 4.5.3) also gives satisfactory results with woven viscose fabrics. Under alkaline conditions in enclosed machines, viscose tends to give problems of reduction with certain sensitive azo reactive dyes. Addition of a reduction inhibitor from the start of dyeing is essential. 4.5.7 Dyeing of bast fibres After appropriate preparation, flax yarns and linen fabrics can be dyed by conventional methods developed for cotton. Residual lignin takes up more dye than the cellulosic component, resulting in a speckled appearance and sometimes an off-tone colour. These deeply dyed specks decrease in depth by desorption of the poorly fixed dye during subsequent alkaline fixation and washing treatments. Every effort should be made to remove these impurities as much as possible before dyeing. In package dyeing flax yarns are more readily penetrated using reactive dyes than by vat dyeing. Dye–fibre reactivity and ultimate fixation efficiency are closer to those of unmercerised rather than mercerised cotton and thus appreciably less than on viscose materials. Careful attention is necessary during the critical levelling and diffusion stage just before and during the addition of alkali. Causticised linen woven fabrics generally give satisfactory results under BATCHWISE APPLICATION
  • 231. 224 DYEING WITH REACTIVE DYES HomeContents normal conditions for cotton. Pad–jig development is regarded as the best approach for the coverage of slubs or other surface irregularities. The pad–batch process is especially attractive for many linen qualities because of the excellent dye diffusion in reasonably short batching times. Careful dye selection is necessary for woven linen that is to be given a crease-resist finish. Limited amounts of ramie are dyed with reactive dyes by methods similar to those for cotton. Bleached jute is batchwise dyed with reactive dyes using salt and careful additions of soda ash. Jute is somewhat sensitive to alkali and control of alkaline pH and temperature conditions is important. Sisal is sometimes dyed with reactive dyes because of their attractive hues, good light fastness and excellent fastness to water, although poor penetration of certain constructions can present problems. Unsatisfactory results are obtained on coir and other bast fibres, however, owing to their deep natural colours and high content of non- cellulosic impurities. 4.5.8 Developments in automation of reactive dyeing Considerable progress has been made in reducing the need for manual intervention in batchwise dyeing processes for reactive dyes by automating the marginal operations. Isothermal process cycles based on automated control, compared with conventional techniques involving portionwise additions of dyes and chemicals, have been devised. The dye, salt and alkali may be added according to predetermined profiles over a given time period and additions are controlled from dyeing programmes stored in the microprocessor memory. This type of control contributes to the high degree of reproducibility between repeat batches that is a prerequisite for the market demands of ‘right first time’ production [62–64]. The modern multi-product injection (MPI) dosing systems enable an exact amount of product to be added to a process bath over a certain period of time by means of an electronically controlled system, including intermittent, linear, progressive or exponential rates [65–70]. Thus the dyer can control the dosage of dyes and chemicals as well as temperature and process time, thereby directly influencing the course of the chemical and physical reactions taking place in the dyeing system. Reproducible dyeings can thus be achieved, since dosing always takes place under identical conditions. Liquid and solid dosing equipment can be connected permanently to the dyeing machine, or may be utilised as a portable unit, servicing several machines [71]. In summary, the MPI dosing system offers the following advantages:
  • 232. 225 HomeContents (1) Total automation of the dyeing process, saving time and avoiding human errors. (2) Ensuring that the necessary chemicals are always available. (3) Increased control and reproducibility of the dyeing process. (4) Improved levelness of the resulting dyeing. (5) Reduced risk of dye aggregation or precipitation during short-liquor dyeing of full-depth colours. (6) More efficient rinsing and washing, since there is less hydrolysis of dye during the process. Methods of control of parameters relating to the physical characteristics of the material to be dyed, taking into account the influence of mechanical and hydraulic forces exercised by the dyeing machine during the process, are now being introduced [72]. A new generation of process control equipment allows the action of these forces in the dyeing machine to be adjusted, not only for the entire process, but even for each stage of the dyeing cycle. Thus the total energy transmitted to the dyed substrate is reduced, with a consequent improvement in surface appearance because there is less fibre disturbance and abrasion. In the processing of yarn packages, mechanical and hydraulic influences may impair the physical properties of the yarn. In order to preserve the quality of the yarn, the pump delivery should be reduced to avoid undue turbulence [73]. The consequent risk of unlevel dyeing makes adoption of MPI dosing essential. Only by the controlled addition of dyes and chemicals at a lower liquor throughput can satisfactory dyeing be achieved. In some instances the turbulence and frictional forces in a jet dyeing machine can adversely affect the surface appearance of the fabric. This reduction in quality can be avoided by reducing the speed of movement of the fabric rope and the rate of liquor flow through the jet nozzle. In doing this there is a risk of unlevel uptake. In order to apply a synchronised dyeing system under jet dyeing conditions [74], it is necessary to determine the kinetics of the dyeing process and to understand the interactions with the following fundamentals of the machine: (1) Circulation pump (variable flow). (2) Transporting reel (variable speed control). (3) Automatic dosing (MPI system). (4) Machine control (fully automatic microprocessor). 4.6 SEMI- AND FULLY CONTINUOUS APPLICATION The lower limit of liquor ratio attainable in batchwise dyeing is about 5:1 or, in SEMI- AND FULLY CONTINUOUS APPLICATION
  • 233. 226 DYEING WITH REACTIVE DYES HomeContents specially designed ULLR equipment, possibly 3:1. Padding methods extend this further to the range 1:1 to 0.5:1. Thus the advantages of enhanced exhaustion and fixation characteristic of short-liquor exhaust dyeing can be significantly improved by adopting semi- or fully continuous processes. Depending on the reactivity of the dyes selected and the temperature of treatment, fixation times can vary over a wide range. Slow fixation is of particular interest to achieve controlled diffusion, penetration and levelness on certain type of substrate, whilst rapid fixation is of greater relevance to optimal economy and productivity. In the early days of reactive dyeing it was soon recognised that the excellent solubility, moderate substantivity and versatile reactivity of the various ranges of reactive dyes gave them great potential in continuous dyeing applications. This promise has been fully realised in the extremely wide variety of process sequences and conditions of treatment now available. These include the simpler single-pad methods in which the dye and alkali are applied together, as well as the more elaborate but versatile double-pad sequences where the dye and alkali are padded separately, with or without an intermediate drying step. In addition to the standard processes for woven cotton, linen and viscose fabrics, more specialised routines and equipment have been devised for the pad–batch dyeing of knitgoods and the pad–steam dyeing of narrow fabrics. The semi-continuous pad–batch process offers a means of producing runs of moderate length to each colour at low capital cost. In effect, this process is an exhaust method at extremely low liquor ratio and ambient temperature in which all the dyebath is entrained within the substrate. The impregnated batch is stored separately from the padding equipment and this allows wide variation of dwell time according to the reactivity of the dyes selected and the pH of the impregnated fabric. Continuous dyeing represents the highest level of productivity, in which the dwell time after impregnation is reduced to a few seconds or minutes by heating the fabric to a high temperature, but this approach entails much higher capital cost. One of the advantages of reactive dyes is that they possess all the attributes necessary for fully continuous dyeing. This can be achieved without unduly sophisticated machinery, as on the pad mangle, drying cylinders and washing range adopted in the successful trials carried out with the first range of reactive dyes to be marketed [8]. In an early investigation of the substantivity and diffusion properties of selected dichlorotriazine, dichloroquinoxaline, aminochlorotriazine and trichloropyrimidine dyes, the fundamental parameters of padding with reactive dyes were interpreted in terms of these characteristics [75]. Dyes of widely differing characteristics do not behave satisfactorily in combination, neither in
  • 234. 227 HomeContents exhaust dyeing nor in padding processes. Dyes developed primarily for exhaust dyeing tend to have high substantivity and to diffuse slowly, whereas low- substantivity dyes that diffuse quickly are preferable for application and washing-off continuously. 4.6.1 Pad–batch dyeing All pad–batch processes follow the following general sequence: (1) Impregnation of the well-prepared dry fabric in a solution of dye and alkali at ambient temperature. (2) Uniform squeezing of surplus liquor from the fabric as it passes through the mangle nip. (3) Wrapping of the batched roll of wet fabric in polythene film and storage at ambient temperature for a specified dwell time (2–24 h, depending on dye reactivity and pH). (4) Washing-off of unfixed dye. (5) Drying of the washed dyeing. The success of this simple but efficient technique is attributable to the low capital cost of the equipment, low consumption of energy and water, excellent reproducibility and scope for selective control of the dye–cellulose and dye–water reaction rates. Batch lengths of 1000–10 000 m per colour, excessive for exhaust dyeing to the same degree of reproducibility but inadequate to justify investment in a fully continuous range, can be processed economically. With moderate and high-reactivity dyes it is essential to employ a liquor feeding device, to bring dye and alkali streams together immediately before the mixed padding solution contacts the fabric to be dyed. Several reliable systems have been devised for this purpose. The initial approach was a stainless steel pneumatically operated tipping bucket into which solutions of dye and alkali were metered separately and mixed en route to the padding trough. More recently, various designs of electrically operated dual metering pumps and valves have been used, whereby the solutions are mixed and delivered to the trough at a known rate of flow [76]. A recent investigation of the use of residual alkali from mercerising to activate the dye solution in the pad–batch dyeing stage was intended to eliminate the need for proportioning pumps and to economise on water consumption by minimising dye hydrolysis [77]. Although high-reactivity dyes may be preferred because they enable dwell times as short as 2–4 h to be used, demands for specific hues, fastness or cost advantages may favour applying low-reactivity dyes at high pH for longer dwell SEMI- AND FULLY CONTINUOUS APPLICATION
  • 235. 228 DYEING WITH REACTIVE DYES HomeContents times, possibly 16–24 h or longer. When dyeing viscose or modal fibres, their different swelling behaviour compared with cotton must be taken into account. On such fabrics and certain qualities of mercerised cotton, dye penetration and surface appearance can be improved appreciably using a slower fixation treatment. Recommendations have been given for the pad–batch dyeing of viscose and modal fibres after silicate-free cold-dwell bleaching [78,79]. Tailing effects accompanying many padding processes arise from the selective affinities of the individual dyes for the substrate. A deeper colour at the beginning of a run progressively ‘tails’ (weakens) to a paler, sometimes off-tone, colour as the run continues. Tailing with reactive dyes is troublesome much less often than with direct dyes. In pale depths under unfavourable conditions, however, preferential absorption can occur until a satisfactory equilibrium has been reached by feeding the trough with an adjusted dye solution. When dyes that differ in affinity are used together they can show selective absorption so that changes in both hue and depth will occur unless corrective steps are taken. Substantivity factors and mechanical causes of tailing have been specified and measures necessary to overcome these problems have been explained [80,81]. For single padding processes with reactive dyes, ‘one dip one nip’ is preferable to a multi-dip and multi-squeeze technique. The simpler system minimises tailing and permits the smallest possible pad trough volume consistent with thorough wetting and impregnation. Running conditions that allow a complete change of liquor in the trough every 2–3 min throughout the run are ideal for most purposes. Mangle pressures are set to give a liquor pick-up of 60–80% at ambient temperature. The Bicoflex IFAS (intelligent flexible application system) can be used to control liquor application. The Bicoflex roll allows the squeezing pressure to be adjusted across the width of the fabric to even out variations in liquor pick-up attributable to variations in moisture content or thickness of the fabric [82,83]. If desired, liquor pick-up across the width can be deliberately varied to achieve novel effects. The CVC padding roller with a continuously variable crown and a controlled pressure nip system can be adjusted to give desired profiles of linear pressure across the fabric surface [84]. Tubular-knitted cellulosic fabrics can be dyed on suitably designed equipment by the pad–batch method, although the capital cost of open-width washing ranges has retarded adoption of these techniques in the knitgoods field [85,86]. The problem of folded edge marking and measures to overcome this have been outlined [87]. Specialised impregnation units introduced for pad–batch preparation and dyeing of knitted cellulosics include those of Beau Tech [88], Jawatex [89] and Calator [90]. Substantial savings of energy, water, chemicals and labour requirements, compared with exhaust methods, are claimed.
  • 236. 229 HomeContents Convincing reasons can be cited in favour of any of the major reactive systems in pad–batch application [91]. Highly reactive dyes can be fixed in shorter times at lower pH and are often easier to wash off, low-reactivity ranges offer higher stability in the padding liquor and mixed bifunctional systems give highly efficient fixation and excellent fastness performance. Detailed recommendations have been provided for systems of all kinds, including chlorodifluoropyrimidines [92], dichloroquinoxalines [61,93] and aminofluorotriazine–vinylsulphone bifunctional dyes [30,31]. Radiofrequency energy has been evaluated to increase the rate of fixation in pad–batch dyeing trials with sulphatoethylsulphone, bis(aminonicotinotriazine) and aminochlorotriazine–vinylsulphone dyes [94]. The dwell time required was reduced from several hours to about 15 min by heating the impregnated fabric in the radiofrequency field. Colour yields with the monofunctional dyes were lower than under conventional pad–batch conditions but the mixed bifunctional types gave equivalent or superior colour yields and washing fastness after radio- frequency fixation. Bis(aminonicotinotriazine) dyeings applied in this way were equivalent to exhaust dyeings under neutral conditions at 130°C, the appropriate control method in this case. 4.6.2 Pad–dry and pad–dry–bake methods With skilled organisation and management, fully continuous dyeing methods offer economic advantages when long runs are required in a limited range of colours. Listing and ending are avoidable and excellent reproducibility is possible, with noteworthy savings in handling and labour costs. The minimum length to each colour on large multiple-units ranges is usually estimated at about 10 000 m, depending on local circumstances and avoidance of excessive downtime. The dye–fibre reaction takes place extremely rapidly in the presence of minimal amounts of concentrated alkali at elevated temperature. With highly reactive dyes the dwell times are sufficiently short to permit fixation simply by passing the impregnated fabric through a conventional dryer. Thorough and uniform preparation of the fabric and presentation to the padding trough and mangle in a crease-free state are crucially important. Where doubt exists about the absorbency of the fabric to be dyed an addition of wetting agent may be necessary. The first pad–dry process originally developed for applying dichlorotriazine dyes together with 10 g l–1 sodium bicarbonate is still important with those and other high-reactivity ranges. Urea may also be added to improve the solubility of the dyes in full depths, as well as salt or sodium SEMI- AND FULLY CONTINUOUS APPLICATION
  • 237. 230 DYEING WITH REACTIVE DYES HomeContents alginate to minimise migration problems during drying. Fortunately, migration in pale depths (where tailing faults are more prevalent) is rarely severe. Migration shows itself as either ‘mealiness’ at the fabric surface or side-to- centre colour differences across the width of the fabric. Such effects may be attributable to faulty preparation, a pad bowl fault or excessive local heating in the dryer. Back-to-face colour differences or ‘twosidedness’, where one face of the cloth is paler than the other, may arise from the fabric construction or be caused by movement of the dye solution towards the source of heat owing to uneven distribution of heat above and below the fabric during drying. For optimum colour yield in the pad–dry process using a hot flue dryer, it is necessary to maintain a moisture content not less than 20%, i.e. a high wet-bulb temperature is required. By control of the air extraction system it is possible to ensure the presence of free moisture in the dryer. Further factors that influence the degree of migration include the swelling behaviour of the substrate, the presence of auxiliaries, the solubility of the dyes and their substantivity and reactivity characteristics. The lower the substantivity of a dye, the greater is the risk of migration. The higher the reactivity of a dye, the lower the migration in the presence of alkali because only unfixed dye molecules are free to migrate. Hence highly substantive dyes containing a high- reactivity system are the least prone to migration problems [95]. Urea is still widely used in continuous dyeing and printing with reactive dyes as a dissolving assistant, a disaggregating agent, a swelling reagent for cellulose and a hygroscopic auxiliary. It is difficult to specify which of these effects is the most important in any given process. Nevertheless, it certainly enhances colour yield, accelerates diffusion and improves the levelness of the dyeing, particularly on viscose fabrics. Although cheap and readily available, urea is not an ideal chemical from the environmental viewpoint and alternatives are being explored with a view to partial or complete replacement of urea by other compounds, such as dicyandiamide [96], trimethyl- or tetramethylurea, or poly(ethyleneglycol) [97]. Soon after the aminochlorotriazine dyes were first introduced, it was recognised that the pad–dry process with bicarbonate, though highly successful for dichlorotriazine types, was totally unsuitable for low-reactivity dyes. Investigation led to the pad–dry–bake sequence, still occasionally of interest where the expensive but more versatile pad–steam range is not available. Typically, the dyes are padded with 10–20 g l–1 sodium carbonate and 100–200 g l–1 urea. Little fixation takes place during intermediate drying and a baking treatment (e.g. 1 min at 150°C) is necessary to complete the dye–fibre reaction. Many viscose fabrics show excessive migration and poor diffusion of the dyes,
  • 238. 231 HomeContents evident as inadequate penetration of colour. These materials can be handled far more satisfactorily by the pad–batch route. A further defect of the pad–dry–bake process is a lack of chlorine fastness from dyes that are normally satisfactory. They become less resistant to chlorine as the severity of the baking treatment increases. The thermal decomposition of urea under dry conditions is the main contributing factor in lowering the fastness of the dyeings to chlorine and also, in some instances, to light. In recent years there has been considerable interest in applying radiofrequency and microwave methods of heating to the fixation of reactive dyes. Radiofrequency treatment offers rapid fixation, energy saving and no increased risk of migration when compared with conventional hot-air drying [98,99]. In microwave fixation, electrolyte addition and bentonite (added as a migration inhibitor) both improved the colour yield, whilst urea accelerated the rate of drying. Aminochlorotriazine, dichlorotriazine and sulphatoethylsulphone dyes showed satisfactory fixation, sodium bicarbonate giving higher colour yields than sodium carbonate or caustic soda [100]. Simultaneous pad–dry–cure dyeing with reactive dyes and crease-resist finishing with cellulose reactants has been of restricted interest, notably for pale depths, since the early years of reactive dyeing. The main drawback of this approach has been the obvious incompatibility of the latent acid curing conditions required for the resin finish with the alkali needed to fix all types of conventional reactive dye systems. The introduction of the acid-fixing phosphonic acid dyes (section 4.3.1) did not offer an effective solution to this problem but the more recent development of the neutral-fixing bis- (aminonicotinotriazine) dyes has revived interest in combined processes of this kind [101]. 4.6.3 Pad–steam and pad–wet fixation methods In the first practical dyeing method devised for reactive dyes, the fabric was padded in a neutral dye solution and then in a dilute solution of caustic soda in saturated brine, before steaming to complete the dye–fibre reaction. This ‘wet- on-wet’ sequence presented serious problems of dye ‘bleeding’ into the alkaline bath even at the maximum salt concentration. Partial hydrolysis of the desorbed dichlorotriazine dye to the chlorohydroxytriazine derivative (Scheme 4.2) resulted in resorption of this inactive species and subsequent loss of fixation. This unsatisfactory sequence was soon replaced by the ‘wet-on-dry’ technique, immediately found suitable on the pad–steam equipment already well established for vat dyeing. This process followed the sequence: pad(neutral dye solution)– SEMI- AND FULLY CONTINUOUS APPLICATION
  • 239. 232 DYEING WITH REACTIVE DYES HomeContents dry–pad(caustic soda in brine)–steam–wash. After intermediate drying in a hot flue, highly efficient absorption of the alkali/salt liquor is achieved. Where small- capacity troughs are fitted, few colour-bleed problems are encountered. Recent research has established that vacuum extraction of the padded fabric is a viable alternative to intermediate drying. It restricts bleeding of the dye into the chemical pad solution and the wet vacuumed fabric retains sufficient alkali/salt liquor to give satisfactory fixation at the steaming stage [102]. Several designs of roller-type air-free steamer with a cold water exit seal are suitable. Each commercial range of dyes requires an appropriate set of steaming conditions. It has been claimed that mixed bifunctional reactive dyes of the aminochlorotriazine-sulphatoethylsulphone type are more reproducible than monofunctional systems because they tend to be self-compensating with regard to the changing conditions of treatment on a continuous dyeing range [44,103]. The advantages of the Vald Henriksen Tric-O-Steam pad–steam range for the continuous dyeing of tubular-knitted cotton fabrics with reactive dyes have been described. The Tric-O-Dry tumbler avoids shrinkage by circulating the fabric repeatedly via a current of hot air through a channel where it is compressed and a larger chamber where it relaxes again. The Tric-O-Steam pad–steam–wash sequence is claimed to be particularly suitable for fixation of aminofluorotriazine dyes [104]. Selected sulphatoethylsulphone dyes are suitable for the pad–wet fixation sequence: pad(neutral dye solution)–dry–impregnate(alkali/salt)–wash. This is of particular interest where steaming equipment is unavailable or fully occupied by other requirements. Caustic soda in sodium silicate solution is usually preferred as the chemical development bath, typically for 5–15 s at 95–100°C. A specially designed wet-fixation trough (with indirect heating to avoid dilution via condensation) is recommended for the boiling alkali treatment. A one-bath pad– dry–steam route for these dyes requiring only 1 min steaming time at 100–102°C was recently described, to minimise migration and avoid problems associated with chemical pad methods [105]. 4.7 WASHING-OFF AND AFTERTREATMENT Effective washing after reactive dyeing is crucially important. At this stage the substrate contains unfixed hydrolysed dyes and usually some residual active dyes. The dyeing does not show optimal wet fastness properties until this loose colour is removed or rendered insignificant in amount. It is remarkable that, on average, only about 0.003% dye on weight of substrate will produce a stain equivalent to a grey scale rating of 4. The amount
  • 240. 233 HomeContents remaining after washing must be sufficiently small to ensure that after migration the fibres on the surface of the material still exhibit acceptable fastness. During washing the rate of diffusion out of the fibre is retarded by the substantivity forces between dye and cellulose molecules. The smaller the proportion of unfixed dye and the weaker these substantivity forces, the easier is the removal of the unfixed hydrolysed dye remaining. Thus important objectives of the design of reactive dye molecules include a high degree of fixation efficiency and, if practicable, a relatively low or moderate substantivity in the hydrolysed form. Informative reviews of the principles of washing techniques for reactive dyeings are available [106,107]. 4.7.1 Batchwise washing processes All washing processes are essentially techniques for achieving progressive dilution, but the effect of substantivity forces on the rate of removal must always be borne in mind. For example, if an exhaust dyeing of a low-reactivity dye is carried out at a temperature between 80°C and the boil and then given an initial rinse at ambient temperature, this will be inefficient because the dye is more substantive at 20°C than at the dyeing temperature. Hot rinsing at 60–70°C is recommended for optimal desorption of hydrolysed bis(aminochlorotriazine) dyes such as Procion Navy H-ER (Zeneca) (Figure 4.10) from cotton yarn dyed at 80°C [108]. An important function of the initial rinsing steps in a batchwise washing sequence is to lower the total electrolyte concentration by progressive dilution and thus to lower the substantivity of the residual loose dye. This dilution, as well as an increase in temperature to the boil for the first washing stage, will greatly enhance the rate of desorption of loose dye into the washing bath. Reducing the carry-over of entrained liquor between successive steps of the sequence is another highly effective means of increasing the washing efficiency. Many enclosed dyeing machines operating at atmospheric pressure are incapable of operating close to 100°C without cavitation developing at the pump, which interferes with effective circulation of the wash liquor. The highest temperature attainable within these limitations should always be used at the washing stage, so as to maintain a boiling temperature as consistently as possible. It is usual dyehouse practice to add a small amount of suitable biodegradable anionic or nonionic detergent. Such agents should be low-foaming and insensitive to hard water. Washing efficiency is often checked using a small sample taken from the dyed WASHING-OFF AND AFTERTREATMENT
  • 241. 234 DYEING WITH REACTIVE DYES HomeContents material before unloading the machine. This is placed between two layers of white absorbent cotton fabric and pressed with a hot iron. The stain on the interior surfaces of the white adjacents is an approximate indication of the degree of staining likely to arise in wet fastness tests on the finished dyeing. The decision whether further washing is needed to meet target quality depends on this result. This test will prove more reliable if the excess of entrained wash liquor is first removed by mangling or hydroextraction, so that it accurately represents the bulk dyeing just before the final drying. In a jig vessel only relatively small volumes of wash liquor can be used at each washing step and the only mechanical expression is provided by the heavy ancillary rider roller running on the batch of cloth as it is wound onto the draw roller. In a recent investigation of jig washing conditions, hydrolysed forms of the high-substantivity bis(aminochlorotriazine) dye Procion Red H-E3B (Zeneca) (Figure 4.11; X = chloro) and its much less substantive monofunctional analogue Procion Red H-3B (Zeneca) (Scheme 4.1; CI Reactive Red 3) were compared in terms of characteristic properties and response to process variables such as temperature, liquor ratio and auxiliary additions [109]. In circulating-liquor machines the dyer has to operate blind, except for the sampling of liquor as it is being returned to the expansion tank. Careful development work is necessary to establish an effective washing sequence that can be programmed for automatic control. Electrolyte removal is essential before efficient desorption of unfixed dye can take place. In-to-out flow cycles are favoured, but if flow reversal is adopted, the in-to-out direction is preferable for draining before bath changes. If the machine can be pressurised it is advantageous with highly substantive dyes to soap at temperatures up to 110°C. The Thermoflush technique was developed by Bayer AG and Thies Maschinenfabrik for the intensive washing-off of reactive dyeings. Once the dyebath has been drained, water is injected without circulation to overflow rinse. A steam treatment then causes unfixed dye to migrate to the surface of the material. By repeating these water/steam injection cycles several times, virtually all the hydrolysed dye diffuses out of the fibres and is rinsed off immediately [110,111]. A further refinement to shorten the process time is the Thies Combined Cooling Rinsing (CCR) system, in which the cooling water passing through the heat exchanger is fed by the liquor pump into the process liquor circulation. The increased liquor volume is taken to drain via an overflow device to maintain a constant liquor level. This combined cooling and rinsing principle may be used when cooling from a hot alkaline fixation step or from a soaping treatment at the boil [64].
  • 242. 235 HomeContents 4.7.2 Continuous washing processes Continuous washing is usually carried out in an open-width washing range. Each vessel has a nip at the exit to minimise the carry-over by the fabric from one bath to the next. The first three boxes of an eight-box range may be used for initial rinsing, the next three for hot soaping and the last two as a final rinse. The concentrations of desorbed dye and electrolyte gradually increase at a rate dependent on the tank volume, the inflow of clean water and the carry-over of contaminated liquor from the preceding vessel. It is necessary to ascertain the degree of dilution of electrolyte by rinsing before the fabric enters the boiling detergent baths, in order to ensure adequate fastness of the finished dyeing. Vacuum extraction has been evaluated as an alternative technique to assist the continuous removal of unfixed reactive dyes from cotton fabrics, as it offers energy efficiency and other economic benefits. The efficiency of desorption by rinsing the vacuum extraction is dependent on the substantivity of the unfixed dyes present. High temperature and low salt concentration are necessary for effective removal, as in conventional washing processes. Steaming of the wet material permits desorption of unfixed dye into the liquor entrained within the fabric and thus assists removal by the vacuum slot. A procedure based on five warm rinse baths and two steaming treatments prior to vacuum extraction provided a highly effective degree of removal [112,113]. 4.7.3 Washing and internal pH Reactive dyeings that contain monofunctional dyes of the haloheterocyclic type (Scheme 4.2) may be prone to ‘acid bleeding’ of the dyed material. The severity of the effect depends on the structure of the dye in the vicinity of the reactive group. Monofunctional dyes that react by the nucleophilic addition mechanism (Scheme 4.3), on the other hand, form dye–fibre bonds that are less stable under alkaline conditions. It is obvious that such phenomena are dependent on the conditions existing within the fibre phase, especially the pH. This internal pH is related not only to the external pH of the dyebath but also to the electrolyte concentration and the proportion of fixed dye retained within the fibre. For a given amount of fixed dye, the external dyebath pH is reduced by dilution of the alkali but the internal pH within the fibre is reduced even more by dilution of the electrolyte. The magnitude of the difference between internal and external pH increases with the concentration and degree of sulphonation of the fixed dye. Since this effect is associated with the dilution of electrolytes, the internal pH reached in the later stages of washing will depend on the quality of water supplied. In a dyehouse that uses exceptionally pure water (e.g. from WASHING-OFF AND AFTERTREATMENT
  • 243. 236 DYEING WITH REACTIVE DYES HomeContents melted snow or ion-exchange treatment) an unusually low internal pH (2–3) may be encountered, compared with normal conditions of about 0.1 g l–1 background electrolyte (internal pH about 4–5). If the fibre interior is unusually acidic before drying, exposure to heat will cause serious hydrolysis of dye–fibre bonds and consequent loss of wet fastness of the dyeing. 4.7.4 Effect of trace metal ions on washing performance If impure rock salt or sea salt is used for reactive dyeing, or if the raw cotton contains an unusually high calcium or magnesium content, this can cause difficulties at the alkaline fixation stage and especially during the washing sequence. Deposits of alkaline earth hydroxides on the surface of the fibres can adsorb dye, causing spotting problems and poor reproducibility of colour [114]. When washing off, hydrolysed reactive dye is more difficult to remove with hard water than with soft water. The fixed dye molecules retain these divalent cations, which then retard the outward diffusion of the hydrolysed dye anions. For these reasons soft water should be used for dyeing and aftertreatment processes wherever possible. Polyacrylate-based sequestrants should be added and washing liquors made slightly alkaline [115]. Washing procedures for reactive-dyed garments that contain metal accessories should take account of the risk of interaction with trace metal ions. Contact of the dyed cotton with press-studs made from polished coppered brass may result in discoloration, since certain o-hydroxyazo chromogens will form copper complexes that are significantly different in hue from the unmetallised structure. It is impractical for the fabric dyer to select dyes that are insensitive to this influence, since the use of such press-studs in garment manufacture may not be predictable. Mild detergents should be used in washing and perborates or other oxidative agents should be avoided. The washing instructions for studded garments should reflect these limitations. The acidity of rainwater and water supplies in some areas may aggravate this problem by promoting the dissolution of traces of copper ions from the studs [116]. 4.7.5 Aftertreatment of reactive dyeings When applying reactive dyes in full depths, especially on closely constructed fabrics or on inefficient washing equipment, difficulties may arise in removing the unfixed dyes. In these circumstances it may be found necessary to aftertreat the dyeing with a cationic fixing agent of the type often used on direct dyeings. These agents operate by electrostatic association with sulpho groups in the dye
  • 244. 237 HomeContents molecules, forming an insoluble dye–agent complex. This is retained by the substrate during washing at temperatures up to 60°C, so that staining of adjacent white material in wet tests is minimised. After the most effective washing treatment has been given, the dyeing is aftertreated in a solution of the cationic agent at the optimal temperature for the specific product selected. If a cationic or nonionic softening agent is applied to dyed knitgoods, the dye-fixing agent may be applied from the same bath. Treatment with a cationic agent is not a substitute for thorough washing to remove unfixed dye. Formation of the insoluble dye–agent complex on goods that have not been thoroughly washed will lead to unacceptable fastness to wet rubbing and possibly other deficiencies. Certain reactive dyes exhibit lower fastness to light following a cationic aftertreatment and changes in hue or brightness may occur. Apart from inadequate washing, another possible cause of staining with dyeings of certain high-reactivity dyes is bleeding under mildly acidic conditions, either during storage in a humid acidic atmosphere or after exposure to a mildly acidic solution. Treatment with an aliphatic polyamine was originally devised to minimise problems of acid hydrolysis on storage of certain sensitive dichlorotriazine dyes such as CI Reactive Red 1 (Scheme 4.1) in medium to full depths. The polyamine is incapable of complex formation with unfixed hydrolysed dye. It is effective only by reacting with a residual active chloro substituent on the triazine ring of the fixed dye molecule. It is not always easy to decide whether unwanted staining, marking-off or bleeding has arisen from inadequate washing or from acid hydrolysis. It may be necessary to counter both possibilities by first applying the polyamine before soaping and then aftertreating with a quaternary fixing agent as described above. 4.8 STABILITY OF DYE–FIBRE BONDS Since the chromogens used to synthesise reactive dyes exhibit poor wet fastness in the unfixed state, the high wet fastness of reactive dyeings depends almost entirely on the resistance of the dye–fibre bond to the agencies characteristic of wet fastness tests, including pH, temperature, surfactants and oxidants. When the dye–fibre reaction product is formed during the dyeing process (Schemes 4.2 and 4.3), many of the factors responsible for the susceptibility of the reactive system to hydrolysis remain operative and may play a decisive part in determining the wet fastness attainable [8]. Thus when a dichlorotriazine dye reacts with a cellulosate anion, one of the electronegative chloro substituents is replaced by an electron-releasing cellulosyl grouping that is less electronegative STABILITY OF DYE–FIBRE BONDS
  • 245. 238 DYEING WITH REACTIVE DYES HomeContents but the other electron-attracting chloro and three nitrogen atoms remain, activating the heteroaryl ring to nucleophilic attack by hydroxide ions (Scheme 4.4). This can result in either stabilisation or rupture of the dye–fibre bond. A somewhat different situation exists for the less reactive aminochlorotriazine dyes. Since amino groups are more strongly electron-releasing than the cellulosyl grouping, the activating influence of the three nitrogen atoms predominates and only the latter grouping is subject to displacement by hydroxide ions (Scheme 4.5). Therefore, this type of dye–fibre linkage will be marginally less stable under alkaline conditions than that formed by reaction of cellulose with a dichlorotriazine dye. N N N NHD OH Cl N N N NHD O Cl cellulose N N N NHD O OH cellulose OH– OH– Partly hydrolysed dye (D = dye chromophore) Dyed fibre Stabilised dyed fibre Scheme 4.4 Scheme 4.5 N N N NHD NH O OH– Dyed fibre (D = dye chromophore) Hydrolysed dye aryl cellulose N N N NHD NH OH aryl + HO cellulose If a pyrimidine ring forms the basis of a chloroheterocyclic reactive system, the effects are similar to those for chlorotriazines but activation by the ring nitrogen atoms is much less and thus the dye–fibre linkage is more stable to alkali than either of the chlorotriazine systems. The rate-determining step for acidic hydrolysis of haloheterocyclic systems is nucleophilic attack of the protonated heterocyclic ring by water molecules. A recent series of quantum mechanical calculations on cellulose dyeings prepared with 12 different reactive systems containing a pyrimidine or s-triazine ring yielded rate constants for hydrolysis under acidic and alkaline conditions [117].
  • 246. 239 HomeContents When fixation to cellulose is achieved by a nucleophilic addition mechanism (Scheme 4.3), an important factor is the reversibility of formation of the dye– fibre bond. Severe alkaline treatments are capable of rupturing this linkage to regenerate the unsaturated reactive system, which may react again either with cellulose or with water (Scheme 4.6). The presence of a less electronegative activating group than sulphone, as in the vinylsulphonamide dyes, increases the stability of the dye–fibre bond but reduces the reactivity of the dye. SD O O CH2CH2OH SD O O CH CH2 OH– Hydrolysed dye (D = dye chromophore) Vinylsulphone dye SD O O CH2CH2Ocellulose Dyed fibre The kinetics of alkaline hydrolysis of a series of fixed vinylsulphone reactive dyeings on cellulose have been investigated at 50°C and pH 11. Bimodal hydrolytic behaviour was observed under these conditions, the reaction rates being rapid at first but becoming slower as the concentration of fixed dye remaining was decreased. These results were attributed to differences in the degree of accessibility of the sites of reaction of the dyes within the cellulose structure [118]. In an interesting recent comparison of dye–fibre bond stabilities over the pH range 3.5–10, cotton dyeings of the aminochlorotriazine-sulphatoethylsulphone bifunctional dye Sumifix Supra Brilliant Red 2BF (NSK) shown in Figure 4.12 were compared with those of the two monofunctional analogues in which, before dyeing, either the aminochlorotriazine group was replaced by aminohydroxytriazine or the sulphatoethylsulphone group was replaced by hydroxyethylsulphone. Under acidic conditions the bifunctional dyeing showed higher stability than the vinylsulphone dyeing, which in turn was more stable than the monofunctional chlorotriazine analogue. At alkaline pH, on the other hand, the chlorotriazine analogue and the bifunctional dyeing were virtually identical in stability, both being markedly more stable than the monofunctional vinylsulphone [29]. For similar reasons, bifunctional reactive dyeings of the aminofluorotriazine-sulphatoethylsulphone type (Figure 4.13) show better all- round stability to acidic and alkaline conditions than analogous monofunctional Scheme 4.6 STABILITY OF DYE–FIBRE BONDS
  • 247. 240 DYEING WITH REACTIVE DYES HomeContents dyeings based on dichlorotriazine, aminochlorotriazine, aminofluorotriazine or vinylsulphone systems [31]. The hydroxide ion is not the most active nucleophile with which the dye–fibre bonds in reactive dyeings have to contend. Many commercial detergent formulations contain sodium perborate or percarbonate that release peroxy species in washing treatments. The perhydroxide anion (HOO–) is an exceptionally powerful nucleophile capable of attacking certain types of haloheterocyclic reactive system to give unstable products. Studies of the reaction of alkaline peroxide solutions with dissolved reactive dyes have shown that reactive chloro substituents are readily displaced. Heterocyclic dyes containing other leaving groups, e.g. fluoro, and systems that fix by nucleophilic addition, e.g. vinylsulphone dyes, generally do not show perhydroxide formation. Surprisingly, however, dyes of the chlorodifluoropyrimidine type (Figure 4.5) readily form a perhydroxide derivative that leads to cumulative damaging effects on dye–fibre bond stability. A comparison between seven different haloheterocyclic systems each attached to the same chromogen (phenylazo H acid) demonstrated several important conclusions [119]. (1) Only dye–fibre linkages that carry on the heterocyclic ring an electronegative substituent that is ortho or para to the dye–fibre bond show both peroxidation and bond breakage, e.g. 5-chloro-2,4-difluoro- pyrimidine, 5-cyano-2,4-dichloropyrimidine, 2,4,5-trichloropyrimidine and 2,3-dichloroquinoxaline dyes. (2) Dye–fibre linkages that carry on the heterocyclic ring an electronegative substituent that is meta to the dye–fibre bond may show peroxidation but only slight breakage of bonds, e.g. 2,4-dichloropyrimidine and dichlorotriazine dyes. (3) Dye–fibre linkages with no electronegative substituents on the heterocyclic ring show neither peroxidation nor bond breakage, e.g. aminochloro- triazine, bis(aminochlorotriazine) and bis(aminonicotinotriazine) dyes. The relationship between dye–fibre bonding and light fastness was examined for ten sulphatoethylsulphone reactive dyes on cellulose and it was shown that the stronger the bonding between dye and substrate, the more stable was the dyeing when exposed to light [120]. Light-fading studies on cotton dyeings of five trichloropyrimidine reactive dyes revealed that the rate of photodegradation of the cellulose was influenced by the presence of the dye. Compared with an undyed control, the blue dyeing was degraded more rapidly but the yellow, orange, red and green dyes tested exerted a protective effect [121]. Repeated perborate oxidation of dyed cotton and viscose yarns
  • 248. 241 HomeContents demonstrated increased tendering of dyeings containing metal-complex reactive dyes. Viscose yarns were more significantly affected than cotton, possibly owing to the lower crystallinity, greater accessibility and higher carboxy content of viscose fibres [122]. 4.9 REACTIVE DYEING RESIDUES IN WASTE LIQUORS As already discussed in the introduction, approximately half of the cost of a typical reactive dyeing may be attributed to the washing stage and treatment of the resulting effluent. A central objective of research into novel reactive dyes and dyeing processes has been the improvement of fixation efficiency and minimisation of the proportion of applied dye that is lost by hydrolysis in the dyebath or by absorption, hydrolysis and removal again at the washing stage. The contribution of hydrolysed dye to the overall behaviour of reactive dyes is most evident at the washing stage and in contamination of dyehouse effluent. With dyeings of highly substantive dyes, boiling water treatment is not fully effective in removing all the hydrolysed dye present. If dye concentration and COD values are determined for the successive washing baths, it may be advantageous to separate the waste waters into high- and low-load reservoirs for possible recycling [123,124]. Reactive dye hydrolysates are not easily adsorbed by sewage sludge in a biological clarification plant. It is desirable to decolorise the liquor at source if practicable. A post-scouring step designed specifically to remove hydrolysed dye as effectively as possible offers the possibility of concentrating the waste dye residues into as small a volume as possible, making water treatment of low-load l